When secondary ground fault transformers were first introduced, some transformer manufacturers required a dedicated neutral, but I don't think they do anymore. We generally still request a dedicated neutral.
You can put as many neon signs on a circuit as that circuit will legally carry, which in the case of neon for some reason or another (600.5(B)(2) is 30 amps. Fluorescent and incandescent are 20 amps (600.5(B)(1).