How many cylinders can you run off 1 hydraulic pump?

I have a hydraulic bed that I am going to redo, which is going to require some more cylinders. I have heard of cars that run off of one pump, but is this reliable and safe? What I also have heard is that you can run as many cylinders as you want, but you just have to have enough oil in the tank to supply them all. Is this true?Please any info would be great