It's pretty simple really.
Once you remove the fact that controllers need to be posted every few years, go on exercise & deployments, go on various courses, do shoots and fitness tests etc - you don't need anywhere near the amount of people. Of course the driving factor in this is the train-to-train situation.
Once it's settled under AsA there will be very little training going on, and everyone rated to normal traffic with no supervision.
Making a roster for two positions (even three with a supervisor) is not hard for 16 hours a day. You could have a core team of 6 doing 2x Mornings, 2x Afternoons and 2 days off. Then you could supplement with one or two doing regular M-F 9-5 shifts.