How much do a 94 van a/c oil charge need if all the freon came out and I need to charge it?

Have you fixed the leak yet? What was replaced? What kind of van? Has the system been thoroughly flushed and evacuated with a vacuum pump? LOTS of variables here... Do you have a set of manifold guages to check pressures?

Oil and freon charges are two distincly separate issues.

All I would like to know is about how much oil the system holds.. or about how much should be put in the system when charging with the 134a... the 134 is enpty... 94 chevy van 1/2 ton..

A new compressor, depending on the model, may need around 6 oz. You need to make a judgment call on how much you think is left in the system (i.e. compressor and receiver) and go from there.

And of course empty the compressor will not turn on and you will have to wire set it to run to be able to add the 134a.
But as noted, if it all has leaked out, the new may just go right out also.