It is usually like 2 or 3 and they will strike together. Having multiple keeps the unions in check and arguing for a pay raise for 60% of workers and nothing for 40%. I that case, the 40 would form their own union
In countries where meta-strikes are legal, the unions cooperate on basic rights, and strike together when needed. Such coordination is explicitly illegal in the US, and has been since the '30s (IIRC).
