how many do i need for 4-8-4's?
Generally three: Engineer, fireman, head-end brakeman.
Les
In passenger service you only had the engineer and firemen.
That may depend more on what's behind the tender. If there were a lot of sealed express cars or mail storage cars, the head end brakeman may still ride the cab.
one with mix freight cars one with passenger cars
I think a mixed train would have three. Engineer, fireman, head-end brakeman.
You would still typically have a five man crew on a mixed train. The conductor and rear brakeman would normally ride in the coach/combine. The headend man might also ride there if the train was typically short. If enough freight cars were assembled to make a decent train (say 10+ cars), then he would likely ride the cab to be close to the tender for coupling and uncoupling at the head end.