If you're just changing the fluid and filter, it's usually about 4 quarts if the transmission is already at the proper level. It uses Dexron II. Don't get the later versions of Dexron because they are of the wrong formulation for the older transmissions. Same goes for the brake fluids. Don't use anything higher than DOT 3.
The dipstick will usually be stamped with the proper fluid brand and type. Our old Grand Prix and Grand Am both had the Dexron and type to be used embossed into the dipstick. So did my Dodge truck.
Something you can do to determine volume would be to capture the old fluid in a good clean container and measure it into a gallon milk jug. That would give you a starting point. Don't remove all the bolts from the pan. Just loosen them enough that you can break the pan loose with a rubber mallet or dead blow hammer. The flange on the pans aren't that strong so if you stick a screw driver in and try to pry it loose from the trans may end up bending it and causing a leak. A putty knife inserted between each set of bolts might help to break the gasket loose.
I've changed my brother's fluid once and I think I recall that there may be 2 bolts that are a different length than the others, so you might want to pay attention to see if there are and where they go. I think it's the one at the center of the pointed end and the other is at the center of the flat end.
