I think this has been changed to a single orbital that gives a 50% bonus now. So you can no longer build more then one.
As far as I know you can still upgrade the Orbital Academy to, for example, the Top Gun Academy, and then build another Orbital Academy on another core world. But there are a limited number of upgrades, so you can't build them on colonies anymore and you can only build a few on your core worlds. Plus the hit point bonus applies only to ships built on that core world, negating the exploit that got players (and AIs) to hundreds or thousands of hitpoints.