Once that parliament is elected, and is begun. Prior to that, they are there for their party. So, on election day, the key number is 323.
The tradition (I cant recall if its law or not) is, there will be another member of the Speakers party as one deputy, and two from another party - so that the loss of seats is balanced. But seeing as there will be more than two key parties after this election, they could try and nominate a LibScum for one of the 'other party' roles, meaning the required majority down to 320!