If it's something that I will be carrying, no less than 100 during the first range session. Some call it break-in, I call it a shake down. Whatever you call it I want to be as sure as possible that it functions. After the first 100, I'll fire a box (usually 20 rounds as it is now sold) of a few different candidates for CCW use. If all work, I chose my favorite, and run another couple boxes through it. Again, just to make sure.
If it's a "range toy" I'll probably go through around 100 rounds during my first range session. All guns are cleaned, and lubricated after every trip to the range.
CCW firearms are wiped down, and inspected at least monthly. They also get some range time with every trip. Not every one, every time, but anything I carry will get a regular run through.
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!