Do you have to add a subfloor to subfloor of a bathroom before adding backer board?

Not usually, no. As long as the sub floor is solid, without water damage, etc then a backer board is usually plenty.

You need to have 1 1/4" of sub floor and backer board when you put the tile down. Generally speaking, homes are built with at least a 3/4" sub floor and 1/4" cement backer board with 1/4" x 1/4" trowel of thinset equals this.

If you are not sure, don't assume. Find a vent in the floor. Pull out the register vent and use a hand mirror to see the sub floor. Take a tape measure and measure its thickness.