The Tories need to vote with her, if they don't they need to resign, it's one or the other. She then needs I think about 80 votes to get it through from the rest.
If she doesn't get it through, which is a horrifying thought it can go may ways.
Small defeat she tries again with some minor tweaks, and with the evidence the pound started crashing hard so pressure to vote with her.
Major defeat and she either resigns, unlikely in my view, or there is enough letters of no confidence sent it there is a leadership challenge. Again unlikely as who woild want to take leadership at this time.
I doubt she will call a general election, unless she if forced ie not enough MPs to govern.
She may take it to thr people, a second referendum, as she knows what no deal will do to thr country and it's odds on the public will vote to stay, but sadly there is too many people who will vote to leave without understanding the impacts, and just mimic parliament, so it's risky.
Basically no one knows what will happen if it doesn't get through,let's all hope it does. Because if it doesn't it's not going to be pretty,