I'd say a maximum of four or five, but I'd like at least one person aside from myself to be part of Control, either a Captain, a soldier or one of the training soldiers at the camp. Or any connection to control at all, if necessary. Even another Lieutenant from a different wing of the Imperial army. You get the idea, mm?
No rules aside from the generic. Bear in mind that Control are trained en masse and are then handpicked and tested vigorously before they are enrolled as part of the unit. They are tough fighters and very, very skilled. Highly-trained soldiers are hard to fight, but they are not invincible. Similarly, the thieves are not defenceless, but their snipers are gone and they have been taken unaware and surrounded.
Just be realistic, is all~