I agree a promoted bestman is still a beastman, and the warband list allows a up to 3 of them... so I've allways played using the max number as reference for the whole band, not only the henchmen. So only 3 beastmen for the possessed, only 1 ogre for ostlanders, only 5 flagellants for witch hunters... even if one of them gets promoted. Maybe some people have a different house rule, but I've allways played that way.
Only exception would be if you get an extra henchmen or hero using Monkey's Paw or Magic Lamp, wich rules specifies you may have more members of the same kind than your warband list allows.