The answer is simple.... As many rounds as it takes you to trust that pistol.
For instance, you might purchase a cheap pistol and find it FTE's about 1 time in 100, causing you to never trust that pistol for defensive use. Then you might buy a new Glock and be confident after just 100 rounds. Or you might get a gun from a friend that you know has a great history, and trust it immediately.
In my case I have a fair number of handguns. Each gun has required a different number of rounds to earn my trust in its reliability, but none of them has required a particularly large number (100 for the least, 400 for the most). Even the two 60-year-old beretta's my dad has loaned me gained my confidence with a fairly small number of rounds.
What is more important is how many rounds it takes me to trust myself with a particular gun for defensive carry or home defense use. This number is almost always much higher than the number of rounds needed to trust the gun itself. The only exception would be when the guns are essentially the same, such as when I already owned a .45 XD and then picked up a 9mm XD. In that case I trusted myself at the same time I trusted the gun.