I think they have mentioned wanting to work on two games at once, haven't they?
It wouldn't shock me if they take 300-350 people to start to build Baldur's Gate 4 and then take the remaining 50-100 to build a Divinity game - then they could staff up as they see fit for both teams. They could keep Divinity more of an AA game and release a new one every 2-3 years, then release a new Baldur's Gate or larger franchise game every 4-6 years.