Ten in the mag., one in the chamber. Anything else and you're breaking some law. Unless you're using the .40 calibre magazines, which usually fit and feed 12 rounds of 9mm (plus one).
Seriously, depends on many factors, but the Sig Sauer P226 generally should be a life-time purchase for you. It is a service pistol, used all over the world by organizations who either stand around looking cool and never un-holster besides an annual qualification, to those who practise intensely and crawl through muddy jungles and every other harsh environment and still expect the thing to go bang and shoot true the first try when an enemy comes around the bend.
How many rounds before the barrel rifling wears out? Tens of thousands, but it is affected by the ammunition type and loading, how often you clean it (and how you clean it), and whether you constantly fire through two dozen mag.s without a cool down. How much use to weaken many of the springs, that's already been given as a recommended 5.000 rounds, and you can buy re-build kits for these consumable items. How much to shake the frame loose, probably never. How much before the finish inside the slide rails starts wearing to a brassy colour and you panic that your gun is breaking, but it's really just normal on a black Sig even with the proper light grease, that's a couple thousand or more.
Not to dissuade you or anyone from shopping used pistols, but stated round counts are always suspect, and should be taken in context with the visible evidence.
