Just install until your phone explodes. It's got some un-defined limit detonation service paired with a seemingly small amount of plastic explosive (C4) in your device. Once the number of third-party apps installed hits some number, poof, your phone will let you know via exploding. During detonation, things like the screen shattering and sending shards of glass into your face, the battery exploding, and the plastic parts of the Evo vaporizing are entirely normal. This is, in fact, covered by HTC and Sprint once the phone is purchased, so no need to worry. Just gather whatever pieces of glass are left from your face, and grab up the smoldering shrapnel that was the edges of the phone, place it in a plastic container, and bring it in. In 3-5 business days you'll have a new phone, awaiting more apps.
In short: one can have as many apps as the phone will hold. Who knows, some people actually need upwards of 50 on a daily basis.