It'll be interesting if the Tories can hold any of them. May give a hint at how much their core vote holds up in the next election. Could be the difference between a Labour government and a Hung parliament. A tory revival looks extremely unlikely.
Several times. The record is 15 in 1986, when all the Unionist MPs resigned to fight by-elections in protest at the Anglo-Irish Agreement. Outside of Northern Ireland, there have been instances of 5 at once and one instance of 6.