It's capped at 6. However, Adrenaline increases your max AP in a given round, meaning that glass cannon gives you 1 more from the duration. Starting with maximum AP means you don't suffer the -1 drawback, either. Adrenaline also restores AP if you're under the maximum, and the same goes for Sacrifice, allowing you to eek out another 3 AP during your turn. Then, there's warlord, for +2 AP per kill. I think that's how you get the most AP right now, but I'm not sure. There might be +max AP on gear that I haven't seen yet.