Skunk, I vote for one spare mag like most other posters but I'd add that I'd replace the weight savings with a Surefire 6Z tactical light in a Blade Tech combo pouch (1 mag, 1 light) or their equivalent and a good folding knife. I'm not familar with the USP compact in .45 but if you have a 10 rd pistol you'd only have 1 rd less than a 1911 or Sig 220 user with 2 spare mags not to mention the fact that I use my light at least once a week and my knife almost daily. In fact, if I choose to carry 4 spare mags or even no pistol at all, I always carry a light and a knife. As life saving tools, I'd venture to guess that chances are greater that I'll need the light to get myself out of an unfamilar building if the power goes out or the knife to cut a seatbelt after an auto accident than needing my pistol let alone the 2nd spare mag. That being said, if you can take the weight and have the real estate to spare, more ammo is never a bad idea.