The UK House of Commons has 650 MP’s, but only 639 can currently vote. Hence the deal needs the support of 320 MP’s to pass, though abstentions can change this number. A defeat by a margin lower than 50 would be seen as a positive, while a margin higher than 200 would be a disaster and weaken PM May significantly. A defeat by more than 100 votes is regarded by many as weak and could unnerve markets.