Don’t get me wrong, I 
Asana but like everything, it has its limits, which is sometimes perfectly fine, especially if you know what/where those limits are 
Most of the below limitations are extreme use cases required to handle large amounts of data, so likely these won’t be an issue to the majority of users.
limitations marked with an exclamation, could not be found in the guide, but are mentioned in the forum, thus the relevant links.
limitations marked with a tick are noted in the Asana Guide but can be easily missed…
So let’s vote here to get a comprehensive list added to the Asana guide - it would be beneficial to have these limitations more accessible/public as they may be deciding factors on HOW (not IF) one may go about setting up large organizations before running into performance & usage issues 
To add to the list, just add your comment below!
1. Custom Fields
Asana Guide link to general Field types and limitations
1.1. Reordering options within a Single-Select or Multi-select custom field
1.1. Reordering options within a Single-Select or Multi-select custom fieldNo longer a limitation!
Not sure if the 25k task limit is precise - it may have to do with more parameters than just task count. I met this limitation closer to reaching only 5k tasks but with tens of projects using the same single-select field which had over 50 options. Whatever the limit, it would also be nice to know if the same applies to multi-select fields.
when adding a new option to a single-select custom field with many options, instead of using the ‘+ Add an option’ button at the bottom of the options list, you can ‘insert’ a new option in-between existing options by selecting the option directly above and hitting ‘enter’.
1.2. Limit of 500 options within a Single-select or Multi-select custom field
Asana Guide link 
Forum closed topic
This number was vastly increased during 2022, currently set to 500 (!!) for single-select options and multi-select options. Asana can put this one to bed for the time being!
1.3. Limit of 30 custom fields within a single project
Asana Guide link Forum closed topic
Currently set to 30 custom fields (plus the 9 default/metadata fields, eg. assignee, due date, created by etc.) This could perhaps do with a bit of an increase for large forms or unique use cases.
1.4. Limit of 30 custom fields within a single portfolio
1.5. Limit of 60 custom fields within a single task
Asana Guide link mentions: You can have up to 60 fields per task (this can be achieved by multi-homing a single task to multiple projects).
You can actually have more than 60 but only up to 60 will be displayed in the Task details panel.
2. Rules
Asana Guide link to general Rule types and limitations
2.1. Limit of 50 Rules that can be setup within a single project
Vote here to increase Asana Guide link
This might be limiting to some users but there are workarounds using multi-homing
2.2. Limit of 20 Triggers per custom rule
2.3. Limit of 5 Actions per custom rule
Vote here to increase Asana Guide link
As a workaround, you can create another custom rule with the exact same Triggers to cause another 5 Actions and so on…
3. Forms
3.1. Limit of 500 questions within a single form (?)
Vote here to increase
Although officially announced at 200 limit, I managed to hit the limit at 500 questions, although there is a warning notification once past the 150 mark that reads ‘Forms with more than 150 questions can be slow to load’ which indeed took about 10 seconds to load, but works!
No reference currently in the guide
3.2. Limit of 20 forms within a single project
At the time of writing, the guide refers to 5 forms per project, although confirmed in the forum to be indeed 20 forms per project as of Dec 2022. Note, multiple forms are only available for Business & Enterprise Plans.
3.3. Limit of 25MB per file uploaded as an attachment to a form submission
Voting here may help!
No reference currently in the guide but confirmed in the forum
4. Tasks & Sections
4.1. Limit of 20 task templates within a single project
Vote here to increase
No reference currently in the guide but confirmed in the forum
4.2. Limit of 30 dependencies linked to a single task
Vote here to increase
No reference currently in the guide. Thanks to @Phil_Seeman for confirming.
4.3. Limit of 20 projects that a single task can be multi-homed to
Vote here to increase Asana Guide link
4.4. Limit of 60 custom fields within a single task
Refer to 1.5 above
4.5. Limit of 50 tasks selected at once to perform multi-select actions
Vote here to increase. No reference currently in the guide.
4.6. Limit of 1000 visible tasks within a section for it to be collapsible
i.e. in List view, a Section cannot be collapsed if it contains more than 1000 tasks that are visible. eg. if the ‘Incomplete tasks’ filter shows less than 1000 tasks, even though there are more than 1000 tasks in the section (e.g completed tasks also), the section will still be collapsible. No reference currently in the guide.
4.7. Limit of 100MB per file uploaded as an attachment to a task/comment
Vote here to increase Asana Guide link mentions: If you have a larger file, consider attaching it from Dropbox or Google Drive. There is no storage limit for the number of files you can upload.
5. Portfolios & Workload
Asana Guide link to Portfolios FAQ
5.1. Limit of 500 projects added to a single Portfolio
Vote here to increase
Asana Guide link and as per release notes of Aug 2022 but see 5.2. below…
5.2. Limit of 250 projects added to a single Portfolio for Workload to function
Vote here to increase, thanks to @lpb & @Bastien_Siebman for noting this.
5.3. Limit of 30 custom fields within a single portfolio
Refer to 1.4 above
6. Dashboards & Reporting
6.1. Limit of 20 items/charts in a Dashboard of a Project
Vote here to increase
Asana Guide link mentions the limit as 15 items, however, you can currently add 20 items/charts.
6.2. Limit of 20 items/charts in a Dashboard in (Universal) Reporting
Vote here to increase
This appears to be limited to 20 items, although there is no reference in the guide.
7. Importing & Exporting
7.1. Limit of 2,000 rows for importing a CSV file into a project (?)
Vote here to increase
Although not 100% confirmed, and no notes currently in the Asana guide, there are reported performance issues beyond this limit. Thanks to @Bastien_Siebman for suggesting this.
7.2. Limit of 140,000 rows for exporting a CSV file from a project
Although technically in the Asana guide under release notes for May 2022 (and here in the forum), this is not mentioned in the guide’s relevant article, here.
7.3. Limit of 2,000 rows for exporting a CSV file from a Search result
Forum closed topic
Unlike 7.2, exports from search results are apparently still limited to 2,000 rows of a CSV, as reported here.
8. Sorting & Filtering
8.1. Unable to sort tasks by assignee if over 1,000 tasks
Forum closed topic
Not sure this one still applies? We have a project with over 5k tasks and can sort by assignee perfectly fine… 
9. File Size Attachments
9.1. Limit of 100MB per file uploaded as an attachment to a task/comment
Refer to 4.7 above
9.2. Limit of 25MB per file uploaded as an attachment to a form submission
Refer to 3.3 above
10. Read-only links (shareable Project views)
10.1. Limit of 200 tasks displayed in read-only List view
No reference currently in the guide
Note, the shared List view displays the first 200 tasks sorted by ‘None’ order, only.
10.2. Limit of 200 tasks displayed in read-only Board view, maximum 50 per column
No reference currently in the guide
Note, that the shared Board view displays the latest 50 tasks added to a column.
10.3. Limit of 100 tasks displayed in read-only Timeline view
No reference currently in the guide
Note, that the shared Timeline view displays the first 100 tasks in sorted by date. Although one cannot scroll to the left or right of the timeline, one can use the zoom controls to Month/Year to display more tasks, these will still remain no more than 100 being displayed.
10.4. Limit of 100 tasks displayed in read-only Calendar view
No reference currently in the guide. Thanks to @Bastien_Siebman for raising this.
Interestingly, this limit applies irrelevant to however many tasks are due on different days or due on the same day. However, if tasks are due on the same day, the most recent tasks added to that day will appear first, before reaching the 100 task limit, therefore possibly only showing some of the total number of tasks on that particular day. 
10.5. Read-only links are only for Organizations (not available for Workspaces)
10.6. Read-only links are not available for Overview, Workflow, Dashboard, Messages or Files
I’m sure there’s more to add so please let me know and help keep this post updated
As noted in the original thread, I’ve been pushing for such a comprehensive limits list for years now; no luck so far. But I love that you started a list with the post you made. I merged the two topics so we pick up the votes from the other one. (Be sure to add your vote, also!)
I will keep an eye on it to add relevant items to this post! 
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