There can be up to 6 mega evolved Pokemon on a field at once. Triple battle, 4-5 dittos, 1-2 megas. Of course this doesn't work ingame and in any competitive group except ordinary link battles with friends. The battle would need to be organized and produced so that both sides would cooperate.
Ingame it is possible to fight almost all double/triple battle trainers with 2-3 megas if ditto is used.