I think if we exclude the preservation era (everything post-1943) then No. 9 is the clear winner in terms of number of railroads operated on. Runners-up would probably include WW&F No. 8 (which was also B&SR No. 3 and KCRR No. 3), and B&B 'Puck' which carried four different road numbers on the SRRR, P&R, and SR&RL, and also operated briefly on the B&SR.