Five: United, American, Southwest, Midwest Express, and Continental.

The only reason I have accounts on AA, WN, and YX is becuase I just couldn't bear to see flight miles go to waste. I've probably flown them a total of 5 times combined in the last three years. The CO membership was forced upon me by my corporate travel dept; in an attempt to get me to switch my loyalty they comped me Platinum. I didn't bite.