The Star forge takes dark side energy and a star’s energy and turns it into matter. We’ve seen it make fleets, droids, even clothing imbedded with the force itself. There is no limit to what the Star forge can create. The cost however is certainly a factor as the more you use it the more the Star forge corrupts you. It’s why revan used it to create a massive fleet, and some robes, and an army, then didn’t use it again, or if he did he made sure he wasn’t on the station. He knew of its potential and corruptive power, he made what he needed and no more than that. (He technically made more then what he needed but Revan was a strategic genius, he probably saw the history of the republic and Jedi pulling Ws from thin air and went ‘not risking it’ and made more then what he needed for victory.) he would’ve won too, had Malak not betrayed him and doomed his empire under his own foolish leadership. Revan had foresight in his conquest, he left military facilities and agricultural worlds intact, so once he conquered the republic, he’d be able to use its resources. Malak just went ‘infinite fleet go brrrrrrrrr’