well the TC holds about a quart of fluid. Once they remove the tranny it should drain out. I also suspect they change the TC in addition to the transmission.

I would buy 6 quarts of ATF. I tend to buy 6 and usually have 1 left over.
Better to be safe than sorry i say.
How much are they charging you for the rebuilt tranny.? Is this through honda?
Also, make sure they offer a warranty.