I've worked in/with datacentres of literally all sizes in my 11 years in IT; I'm not aware of any concrete model or best practice (perhaps there's something in ITIL, which I've never bothered with to date).
In my view it really boils down to your organization's needs; do you have a lot of change activity happening on all or some nights? Do you have a lot of systems that need constant babysitting? Do your end users call directly in with urgent change requests (application restarts, etc.), or problem reports? Can you afford to have the datacentre unmonitored for up to 30 minutes while a lone (and hopefully trusted!) employee takes a meal break?
Weigh your options. If cost is a factor, start with minimum staff, and observe and measure performance.
Although it's not good from an employee morale standpoint, if you have a larger budget, start with a large number of employees and watch their utilization levels. If you find that they spend more time having LAN deathmatches than doing actual work, then you can cut back until you reach your optimum FTE levels.
The short answer: I don't believe that any one formula, even if one exists, can dictate how *your* datacentre should be manned.