There are 650 seats in the House of Commons – as this election was fought before planned changes which would have reduced the number of seats to 600.
So the normal assumption is that a party needs 326 seats for an overall majority. However, in practice, Sinn Fein MPs – four in 2015, but that number is predicted to rise – do not take their seats, and the Speaker does not count.
If the Conservatives fall short of 326, they could either try to form a coalition, arrange a looser “supply and confidence” arrangement – in which another party will support them for Budgets but not necessarily other legislation – or try to govern as a minority. In the case of the latter situation, the key test is whether a government can pass a Budget. If they cannot, it’s effectively a vote of no confidence.
Under the Fixed Term Parliament Act, if no party can form a government, then after two weeks, another election is called. We would go straight to another short campaign.