The government has a majority of four. So if the nine Ulster Unionist MPs were to join the other smaller parties in backing Labour, three Tory MPs voting against the motion would be enough to precipitate a government defeat, assuming all the other parties turned up at full strength. It would require five to abstain to have the same effect, again if all the other parties voted at full strength against the motion.
The Scott debate next Monday - running from 3.30pm to 10pm - is scheduled formally to be on a government motion "that the House do adjourn". Such a motion cannot be amended - but MPs are within their rights to oppose or abstain. Labour and Liberal Democrat whips will direct their MPs to oppose.