I hope you are enjoying your Wednesday so far, I am sorry to hear that you are running into issues here! I can confirm that there is no hard limit to how many Tasks you can add to a Template Project.
Sometimes if a Template Project is quite big, creating a copy of it can take a few moments. Should this be the case with your Template Project, you will see a small number of Tasks at first, however, after a while you should be able to see all Tasks from the Template Project in your newly created Project.
Also, once the new Project has been created you should see a notification in your sidebar which states “[Project Name] was created successfully” and you should also get an email notification confirming that the Project has been copied fully.
Could you possibly try to create another Project from your Template Project and see if either of the above happen for you after some time? If not please feel free to get back to me on this thread and I will help you further with this!