how many lbs. of freon dose a 2.5 home a/c unit hold?

First off are you sure it has freon(r-22)?. it could be any number of other refrigerants. Secondly, how much to charge depends on many things. it needs to be charged by superheat more than likely, and you need a EPA certified tech to legally do that.

I am assuming you are wishing this information for reference only. The information can usually be found on a label that is located on the unit itself. be advised this will be only for the unit itself and does not necessarily include the evaporator coil and connecting lines . A technician will "fine tune" the charge while servicing the unit.