How many staff members do you think a forum of x size should have? Do you think the number of staff members should depend on the overall size of the board, or the current activity of the board? If your forum was once active but activity dropped substantially, would you let staff members go?
There is no definite rule on this. A forum owner can decide to have up to 5 staff at a go as long as each staff member has a role that they will play in ensuring that the forum moves in the right direction.
Also, a forum owner can decide not to hire a staff if he or she feels that the tasks in the group are ones that he can handle without having another staff on the team.
In my view staff should be hired with great caution. It's easy enough to take someone on but not necessarily so easy to let them go. The number of staff might be determined by the nature and activity levels of the forum. I'd argue in general less is more. Better to have one longstanding, active member with the right attitude than a couple of members with less enthusiasm for the forum. I've always found the best staff come from existing forum members but certainly not from anyone who offers themselves up.
To sum up I don't think it's so much about numbers, more about having the right people for the job.