The "old" Tesla factory was bought from a joint GM-Toyota venture that achieved average production of 25 000 vehicles per month running those lines for 25 years. Then that venture failed, Tesla bought the factory and repurposed the existing infrastructure.
It's like if SpaceX started by buying an old working NASA rocket design and repurposed it for the Falcon, but for Falcon Heavy they started entirely from scratch developing rocket fuels, engines, materials etc. without ever having done so before.
Side note: if you know anyone with a Tesla and are in the Bay Area, owners can take three friends with them on a tour of the factory. It's fantastic and I highly recommend it.
How much further can they ramp up? That is the pertinent question.