One starbase (and any allies of yours can also build starbases there -- limit is per player), perhaps a minefield as well courtesy of your Ruiners.
Your starbase could also have a phase stabilizer module if you've done the research to enable defense (say, shifting a defense fleet between wormholes and star), but of course travel through phase stabilizers isn't instant, so if they're quite far away and the enemy fleet is strong enough to rapidly break local defenses you may still have problems. Especially if it's a feint at the wormhole designed to trap your defense fleet in phase space (out of communication until it arrives at the destination) while the main force goes where your defense fleet used to be.
