At the time of writing, the state of the parties (alt source)as it affects this calculation is:
- Conservative: 314
- DUP: 10
- Labour: 247
- Sinn Fein: 7
- The Speaker: 1
- Vacant: 1
- Everyone else: 70
- TOTAL: 650
However, the 3 Deputy Speakers - of whom 1 is Conservative and 2 are Labour - don't vote. Sinn Fein don't take their seats, so are also unable to vote. With the speaker and one vacancy, this reduces the number of voting MPs to 638.
This makes the number of votes required for a majority to be half of this plus one, i.e. 320.
The Government are in a confidence & supply agreement with the DUP, so in the event of another confidence vote, the number of Government votes available is 314 (Con MPs) - 1 (Deputy Speaker) + 10 (DUP MPs) = 323.
The number of Opposition votes available is 247 (Lab MPs) - 2 (Deputy Speakers) + 70 (everyone else, apart from Sinn Fein) = 315.
Hence the Government, in theory, have a majority of 8.
However, for non-confidence & supply votes, these numbers change to 313 and 325, leaving the Government short by 12 votes. And indeed, they have lost a number of votes recently - not merely because of these numbers, but also because, on Brexit issues, a number of Conservative MPs have not voted with the Government.