Not many. It’s one of the oldest tricks in magic and pretty much everybody in the community says “don’t do it don’t do it don’t do it”. People still do it, but it’s a serious advanced trick.
Magic acts usually involve getting the audience to look in the wrong place. In the case of this, dwelling ad nauseum on the preparations overlooks the assumption that a bullet is a bullet. While the projectile is able (hubba hubba) to puncture a balloon on the way to the magician’s mouth to prove there was a bullet, no mention is made of the powder load in the original cartridge. My guess is that the cartridge is underloaded with powder being just enough to get the bullet out of the casing. But that’s only a guess. IANAM.