What is the maximum gpm through half inch copper pipe at 25 psi?

When you say 1/2" are you talking about 5/8 OD or nominal size and which type of copper? K /L/ M as there is friction looses to consider

I am referring to 1/2" nominal type L copper pipe, and the friction loss is negligent because the section of 1/2 inch tubing is only 2" nipples on either side of a electric shut off valve. The actual set-up is 1" PVC supply at 65psi, reduction fitting to 3/4" into a pressure redusing valve, 3/4" out of the valve through another reduction fitting to 1/2" into and through the shut-off valve, then into the furnace. Distance from the first 1' to 3/4" reducer to the furnace is only a distance of about 14". Hope this helps.