Why don't you start downloading apps alphabetically and let us know what letter you get to when it finally crashes.
Just kidding. Under any reasonable and realistic circumstances, you wont come close to filling up your device memory with apps. I'm not saying that it's impossible, but it wont happen unless that's what you're trying to do. Having said that, if you download an app that you don't use, delete it. There no sense in having your phone loaded up with a bunch of junk. It would be like not taking out the trash in your house because there is still some more room left to pile it up in.