2 and constantly mixing it up so my body doesn't get accustomed to training,and to keep my muscles in a constant state of shock to continue their growth.
usually about halfway through I will mix in cable crunches (4 x 20), oblique crunches on the back ext bench, and hanging leg raises.......
I also do about 4-6 sets of standing bicep curls on chest day because my arms lag
I mix up triceps on back day (Tuesday) the same way and then work arms separately on Thurs......
id say 1 or 2, if u dont really count bicept/light forearms a 2 muscle workout, back/abs,and shoulders/light traps, but i do do chest/tricepts which i consider 2 major muscle workout.