I have been involved in several saas projects. I still work with one of them. I have not found any standard ratios. It really depends on several factors including how intuitive is the product? How mission critical is the product to the subscribers daily workflow? What is the learning curve? What is the adoption rate? What is the average customer aptitude for your product, etc?
I can tell you that one client manages about 150 subscribers with one part time support person. I also have a contact that provides web hosting and manages about 800 hosted clients with one full time support person. But, te variables really skew these numbers.
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