I admit I'm not a master of the AI's attacking mechanics, but from what I know of warrior hiding/choking on a forest hill vs archers when worker stealing, you can probably get away with stacks of 1-2 privateers if they have full health, or at least 5.2+. In fact, I can attest to it from playing my last game. I was semi-isolated (and then soon isolated ) on a monarch game, and the only one who ever attacked my uninjured lone privateer stacks was Napoleon. There weren't any other notable risk-taker types in my game though (later, Napoleon risked 4 galleons and 7 caravels on a destroyer of mine in a coastal tile because, although it was combat 5 from its time raiding Napoleon's seas as a privateer, it was "injured" at 29.3 Health)