All the competitions I've entered specify any restrictions like this in their entry/registration guidelines... if none are listed, then email the organizers to make sure. If there are none, enter as many as you like wherever you'd like. Just be careful that you understand whether the limits are on "Category" or "Sub-Category" ... if there is a "2 beer per *Category*" limit, then you are free to enter 2 15A entries... if there is a "1 beer per *Sub-Category*" limit, then you could enter only 1 15A, but could also enter a beer in B,C, and D as well.
I've seen it done many ways... when in doubt, contact the contest organizers. Organizers are given ranking credits by the BJCP for running competitions, and they take it pretty seriously... you should have an answer quickly.