I previously did some testing and posted current measurements results in another thread. It turns out, it's pretty safe to run many pins on one circuit, as long as players don't start triggering magnets at the same time!
Quoted from HighVoltage:I did a bit more consumption testing on some machines (note, all LED'd):
Game, Idle, Playing, Max
WCS (WPC), 1.2, 1.4-2, 4.6 (magnet)
T3 (Whitestar), 1, 1-2, 2.5
ST Prem (Sam), 1, 1-3, 5.5 (magnet)
Black Knight (W7), 1.2, 1-2, 3 (both magnets)