How many quarts does '85 Chevy blazer transmission take

It depends on which automatic trans you have, the 700r4 takes 11.5 qts empty but you won't get that out when dropping the pan and filter, in that case you will need approx. 5 qts. If you have a TH400 then it still holds 11.5 Qts but you will only need approx. 4 qts during a change. Best to put 4 in and run the truck, shift through all the gears and check it running in park. Add as needed, try not to overfill.