Several. I rarely need to refer to the schematic. I disassemble and clean up all the steppers, then tighten, clean, and check every switch, and inspect all the wiring in the game. They usually fire up and then just need a tweak here and there. The score reel switches usually need the most tweaking.
Sometimes there's a hidden wire or two broken off a solder tab that's hard to locate, or an open wire in the bundle that requires breaking out the schematic, but not usually.