I don’t believe there is a limit actually. If you have over 1,000 tasks in a Section you aren’t able to collapse it (that might be specific to My Tasks 
). However, there isn’t a hard limit to the number of tasks you import or that exist in a project.
That said, users have expressed issues regarding lag and the type of CSV files that successfully import when working with a high count (>500) of import lines. Here is of those mentions:
I can’t find the post where a user was performing a test to see at which point his import wouldn’t perform because it reached some unpublished limit like 2,000 or so.