It really depends on the pattern and type of usage.
For managing incidents, we generally advise 3:1 but if you start doing change management and want lots of approvers, then it can go to 6:1.
To assist with managing concurrent license, you can set the server timeout and then if a concurrent user is logged in and does not interact with SD for this period, then their can be grabbed by another concurrent user.
Sorry I should have provided further information. We're using the Incident, Service Call, CMDB, Maintenance Contract and Change Module. We go through over 100 changes per week with approvers. We do not use Service Pages, so all these people are required to login to the GUI. All our help desk staff are named. So it's just for our 2nd and 3rd Level. We're currently at 11:1 and have exceeded our concurrent users a couple times. Hence my question. Any other suggested ratios?
Another thing to keep in mind are the number of different processes to be supported.
For example, in change mgmt., very few users require named licenses, and the remaining users will have a high user to concurrent licence ratio. In incident mgmt., the developers will have a very high ratio, but the 2nd line supporting Windows platform will have a much lower ratio.
In our case, the overall ratio is about 10:1, but we expect that ratio to get lower as our change mgmt rollout continues.