You need only one mountain bike. You need this bike. I've raced competitively for 12 years. I've ridden for sponsors and have been unsponsored. XC, Short Track, Enduro and Marathon. These days, I own a single bike, and this is it. A recent weekend looked like this:
Friday - Race the Pro fat tire crit, all on city streets, at the Grand Junction Off-Road in Colorado.
Sunday - Race Pro 40 mile XC, on arguably the most technical XC in America at that same race.
Monday - Do lift-access runs for hours at Deer Valley, UT on "Expert only, downhill bike required" trails.
During those three days, I never changed a single thing on the bike aside from dropping the seatpost four inches on Sunday. I ride Enve M60 wheels, Continental X-King 2.4 tires, and XT brakes. The bike weighs in at 24 pounds.
I work at Competitive Cyclist, I race the fast races and I do the fun rides. I could have several bikes and I could have any bike, but this is the one and only.