The question How much does on average customer/visitor support account to? is quite general but I could suggest one way you can estimate and then find it out through experimentation:
Support staff is usually a fixed cost you could estimate, e.g. 1000 USD a month.
How many of your paying customers are likely to request support? There is no rule of the thumb, perhaps ask someone who is working in the same or a similar market niche. The hole idea is to estimate some ratios, let's say per month:
#paying customers needing support / #paying customers
Even if you just started, and have 2 customers you could figure out how many need help.
Then, how many customers could 1 support person handle per month? Again, you have to estimate.
Ok, so far you have the %of customer needing support and the crew of supporters that can help them, e.g.: 30 of 100 customers, could be handled by 2 supporters.
Since your Product is a Saas, you should have estimated the churn rate and sign up rate, so you are likely to know how many customers there will be next month, and consequently how many would need support, thus having an estimate of the size of the support staff.
As you start getting customers, you should always measure that %, because if you improve the software and documentation it is likely that the % should fall. If you do not improve it, it is likely that more customers would be in need of support.
Another tip: Consider including selfservice support software, such as tenderapp.com which allows you to redirect most obvious questions to