I don’t know.
Really, there’s no magic number! You should have just as many as your customers need and you can sustain with your current resources. So unless your organization is massive you probably can’t offer more than a handful and unless your customers are really all the same you probably need more than one.
Start with the customers: can you group them into logical segments? (Don’t just guess: ask them!) Perhaps you have smaller, budget customers, with limited needs; larger customers who want round-the-clock support; and more demanding customers who need proactive support, not just a number to call when they need help. That would suggest three support offerings: one with business-hours support with a limited number of authorized contact; one with 24×7 support; and one with an account manager to provide the extra level of care for the more demanding customers.
Don’t go overboard with the snumber of support offerings: more is not necessarily better. If customers (and sales reps!) cannot easily grasp the defining features of each offering and how they match their needs, they won’t buy.