No such maximum limits are there. I have seen BIG Projects created by some PPM customers which are having hundreds of tasks or line items. However, if you really want to know if there is any limit for that then, I assume, you are concerned about the PPM performance. PPM consultants suggest there should not be more than 100 tasks under one project, to make sure that the performance does not get affected. Yes, PPM might still work fine, but it becomes difficult to manage such long list of tasks under one project.
Best practice is to use a scheduler. PPM goes with Open Workbench or Microsoft Project. Now it has been seen that these schedulers suffer from serious performance issues while handling more than 100 tasks because so many tasks means lots of resources, and even more assignment... Phew!!!
See this knowledge document: Opening a project in Open Workbench is taking long - CA Knowledge
Hope this helps.
Cheers!!!