Whatever the primary is and a reload for it. So if it's a J, then 10rnds. If it's the G26, then 11+10. If it's the G17 then 17+16. The spare magazines are more for malfunctions than more ammo, though.
Shot placement is the most important. But as I can't predict how bad it will be AND if it doesn't really inconvenience me alot, I'd rather have the extra just in case.