How much caulking do we need to buy to caulk the rim of a standard 30" x 60" bathtub?
One standard caulk tube will do a tub plus another.
The caulk link above - my go to. The gun is just an example (that brand gun is actually crap and pushes out too much even when no pressure).
The way I caulk a tub is to run a first pass that goes as deep in as possible, hit it with a wet paper towel or sponge (push the caulk in), wait about a half hour, and then do the second coat and smooth the finish.