Depends on timing I would say - with, say, a month to go for the exit date and it being obvious that there won't be a deal - she could probably withdraw the whip from quite a few. At that stage Labour aren't going to try and stop Brexit, the political cost at home would be too high so they might criticise, but they aren't going to try to stop it. So who can these ex-tory rebels vote with to stop it?
in a vote of no confidence, even abstaining would end their careers, and they aren't going to vote for Corbyn to become PM - and neither are the DUP.
She doesn't need a majority of the house to be Tory and DUP, she just needs a majority who don't want to end their careers or want Corbyn to be PM. That's all that matters in a vote of no confidence, she needs 50%+1, where those votes come from, whether signed up Tories and DUP or 'independants' is of no consequence...