I think people haven’t answered this because any answer would depend on many factors“.. How big is the facility? (square feet, number of users, pattern of use) What sort of equipment do you have, and what sort of programs do you offer? Are you planning to hire staff with specific skills, and will they do a specific job, or will you have staff expected to do a variety of tasks? Are you simply referring to how many people you will have on a fitness floor to monitor use of machines? Again, it depends on what your policy is on whether people are allowed to schedule training while they are on the floor, and what their other duties are. Many places separate the fitness staff from the sales staff from the management from the front desk staff.
You know, you might want to talk to IDEA about a fitness manager membership. I believe it includes a magazine designed for fitness business owners, which should have a lot of good resources on matters like this.