What do you mean with more than one being available? As in, the participants can choose one car to mod from several possibilities, or that participants can (must?) mod several cars? My answer would probably be cautionary in either case, but for different reasons. That said, there is no absolute rule against it in the main ARM ruleset - probably because I never considered it.
For the former, I can see it being difficult to evaluate the restomods made to one car to the restomods made to another car. How can you fairly compare potentially completely different restomods and choose one winner? You’d have to come up with some convoluted criteria so that it is not completely subjective.
For the latter - I just want to point you to PDC, the competition series that died exactly because one host decided to mandate a three-car line-up in an otherwise single-car-per-round series. There was one submission, and then the series died. With the niche nature and low uptake of ARM, I fear that it would also go the way of the dodo - or at least result in a failed round.
