Where do my apps go when I run out of screen space?
The iPad can contain up to 11 pages on its home screen, each page containing up to 20 apps (not counting folders). If all the spots are full when a new app is downloaded, the app will download and install, but not have a spot on the home screen. You can run it through search, the fast app switcher, etc., but not from the home screen.

If space is cleared on the home screen, it appears Apps don't fill in automatically. Rebooting has been known to help.

Now that folders allow more than the 220 apps, I'd recoommend making sure you have an empty spot for the app to appear before purchasing.