The full size Glock 21 .45ACP has a 10+1 capacity for the civilian model and 13+1 for the pre-ban and LEO mags. The 9mm will also only have 10 rounds in the post-ban civilian mags. The pre-ban and LEO Glock 17 mags hold 17. The compact and aub-compact versions hold less.

But in reality it's not the number of rounds you have, its how you use them. Most defensive pistol encounters involve less than 4 rounds being fired. Sustained firefights are not very likely, so having tons of ammo isn't really a major concern. Regardless of the pistol you choose, the most important thing from your perspective is making sure that you can hit what you're aiming at. It doesn't matter if you have 15 shots in the end when you miss with the first five and the bad guy kills you before you get off the sixth. You're still dead. So what's really important is choosing a gun that fits you well that you can shoot well. If it's a 9mm, then get a 9mm. If you can shoot a 45 well, then by all means get a 45. A center mass hit with a 9mm you shoot well is far better than missing with the 45 you're a mediocre shot with.

Don't get all wrapped up in velocity and energy numbers, either. A good hit with a 9mm using good defensive JHP ammo will get the job done almost as well as a .45ACP. If I had to rank the most important things to consider in a defensive handgun it'd look like this:

1.) Reliability. Obviously a gun does you no good if it doesn't shoot when you need it to.
2.) Practical accuracy. When I say "practical" I mean it in terms of being the combined ability of both you and the pistol to put rounds on target at ranges typical of defensive encounters (25 yards). Almost all pistols made today are themselves accurate enough out to 25 yards to hit person center mass every time. The major variable is the shooters ability to take shoot the gun. A 2" snubbie isn't necessarily an inaccurate gun, it's just very hard to shoot well.
3.) This depends on what you're using it for. If it's for CCW, then the third most important factor is concealability. The saying goes, "the best gun to have is the one you have with you," is true here. A full size .45 is useless if you left it at home because it was too big to conceal or too heavy to carry around all day. But the .22WMR you always have in your pocket might save your life. If the gun is strictly for home defense, then the third consideration should be firepower. Basically, use the most powerful caliber you can handle and shoot well.

I hope this helps you.

Mike