Luckily, Tesla appears to already be employing two shifts. (Possibly three -- it's not clear.)
From what I can tell, they have not gotten the first shift up to the target rate of 400 / week yet. Honestly, I'm not sure they ever will; those sort of targets can be overly optimistic. But even if they have to settle for 350 / week, with two shifts, they can catch up with the demand.... for now.
And at the current rate of demand, they won't have to lay off either shift even if they do get up to speed, which is nice.
As others have noticed, the problem lies in delivery and service. Everyone's noticed delivery problems, probably due to difficulty scaling up; and Tesla is definitely not ramping up the service centers nearly fast enough (though I'm sure they're going as fast as they know how to). All of Tesla's spare cash is going to have to go into ramping up delivery and service for a while, and those are both expensive processes with very few economies of scale. (Though there are some potential economies of scale in delivery. They're actually getting to the volume level where it would save money to ship cars in batches of 200-400 by rail from California to somewhere east of the Mississippi. The Tesla Factory is rail-connected... but they don't have a rail-connected transload point east of the Mississippi.)