Ultimately, they are going to need 3, and perhaps 4 transporter cars, if they can churn them out every 5 days, as the round trip has taken just over 2 weeks. So I don't think transporters are the limit.
Not that I think that's going to happen - I'll be surprised if they get another car in service in December, to be honest - let alone 4.
Edit - though looks like the next car (4402? 4414?) is ready for pickup, so perhaps they'll get another one in service after all. Still, that's only a single car arriving in Toronto during December. But if correct, this is about a 21 day build following a 28 day build - so that's actually a bit of progress.