There is little guidance. Certainties first. If it's a deputy, the head has a say. But if it is a head, the present head must be kept well away. The latest regulations make that clear.
Second, you must work with the same group throughout the long process and not bring anyone else in. An appointment was overturned by the courts because one chair, faced with one of the team falling sick on the final interview day, drafted in another governor. If someone drops out you have to carry on with the rest, which means you'd be wise to start with enough to survive uch a misfortune.
Personally, I'd go for seven, though some might think that too many. But if you break up some of the final interviews, with small groups each taking one aspect, it works well. People have to be realistic about the time commitment involved: strictly every member must take part at every stage. When I did it, it took 100 hours - more for the chair.
As for the mix, you need a few who are experienced and know the school well but avoid making newer governors feel excluded from this important process. Try not to over-represent one kind of governor. If possible I would favour having a teacher governor. They are eligible as long as the appointment could not create a vacancy for which they (or a partner) might apply and as long as no relative or partner applies.