I assume you're intending to use 120 film? If so, frame numbers are imprinted on the paper backing. Just get hold of some dark red transparent plastic and have a look at the backing to see where the frame numbers for your chosen format is located (6x6 is in the center; if memory serves, 645 is at the top and 6x9 at the bottom, but you'll see once you get hold of a roll of 120...), then drill a hole in the back at the appropriate position, glue the red plastic onto the body and Bob's your uncle.
It would be advisable to have some sort of mechanism to keep the film flat-ish against the rear of the box - for one, it will keep the film flat during the exposure; also, it will ensure that whatever stray light enters via the hole you drilled for the film frame readout does not bounce about the insides of the camera. (Shouldn't be much of an issue - you'll paint the inside black anyway, I guess.)