Hi. I'm a Business Intelligence consultant with most of my customers being call centers.

There are definitely guidelines you can apply but it will be based on several factors. Your question references ratios, which I assume means you would like to know how many agents per customer. That number will vary greatly depending on a number of other factors including:
-what is your ASA target (Average speed of answer)?
-what percentage of calls should meet the ASA?
-are their penalties below a certain threshold (if less than 80% of calls meet ASA in 24 hours, for example)
-how long are calls waiting when they don't meet ASA?
-what is the call distribution by day of week, time of day and holiday v. non-holiday.
-what is the average call duration?
-what is the % of calls requiring escalation or call back versus calls resolved on first contact?

To simplify it though, the two most important (IMO) will be call volume and your target for ASA (assuming you aren't answering then putting them back on hold, etc).

To simplify though, the top 3 are: ASA, Call length and call volume. Regardless of the size of customer base.

A good reporting system that combines live metrics and daily/weekly/monthly analysis will help a great deal.

Feel free to set up a call if you'd like to talk about this in more detail.