So 750 AP is the minimum you should have off table.....( of course when playing a campaign game with a force that has been reduced to say maybe ? 2100 AP ??? ...then you would not have to keep anyone in reserve.
Remember you have to capture secondary objectives with your assault force in order to bring on the reserves. So you cannot put everyone in reserve , but there is no maximum AFAIK.
I'll bring something in, then. It'll be a surprise . If I don't hear different I'll bring in both armies but I'd rather only cart one or the other along ;)