TTC's been reporting and publicizing that this year's schedule was 64 cars in-service, with 21 in Q4. All the cars up to 4457 (except 4401) were in service by December 30, 2017, with car 4458 entering service on January 6, 2018.
If all goes well, when car 4513 enters service, it should be the 56th 2018 car.
This means they only need to offload 8 more cars in 2018. If 4570 and 4571 do arrive, then it's only up to car 4519 that needs to arrive in time, to enter service before New Year's.
There may be some internal targets to off load 10 more cars, but they could miss those last 2 cars, without getting any real flack.
Seems optimistic though to put 10 more cars in service in the next 25 days, even if they do all arrive before Christmas.