Eh, it's kinda dependent. Usually just one extra magazine, however, if I'm going into certain areas, or it's a given time of year or event, I may carry more. For example, I head into certain "Higher Crime" areas, I'll bring 2 extra magazines, and sometimes more. Black Friday, yeah, I have about 3 15 round magazines if I carry one of my 9mm. Or any event that is "prime target" for a given attack. For example, last time I attended "Thunder Over Louisville" I brought 2 extra with me. it was right after the Boston race bombings and I figured there may be a possibility of something happening.
Then, in Louisville, there was a night that many called the Purge. Which, long story short, someone released a lot of propoganda promoting a given date to be a night of massive crime just like the movie. Well, some took that to be serious. I remember that night walking out to check my mail, I had a VP9 on my hip, and a 12 gauge in my hand. Back inside I had a loaded AR-15 ready to go. That said, I had already heard many gun shots that night, and there was quite the show on the police scanners.
TL

R - Depends on the situation at hand.