Not a limit I have seen. However, good practice would always be to download your projects as aia files for backup and safekeeping. If it helps, I routinely clear my AI2 of projects each month (@ 100), backing them all up on my hard drive, for reuse if required.
Again, not sure of a limit, but we saw a couple of days ago a developer who had 56 different extensions in their project. This could indicate a lack of programming/coding knowledge in AI2
On one project ? Some say 7000 blocks per screen. You should not exceed 10 real screens in a project to ensure stability, but we have seen projects with between 30 and 70 screens in the past (but this is not recommended). Again, good programming/coding and use of available blocks can help to keep down your block count.
None of the above should be targets, you should seek to minimise these wherever you can.
Others may have more to add