Do I have to pay for DataMiner client access licenses, is there a cost per user ?
As Ive mentioned, you can have as many named individual users as well as as many concurrent users as you want in your DataMiner System, i.e. there is no license constraint related to those metrics. So you can really ensure that as many people as possible in your organization can benefit from being able to tap into the wealth of data that DataMiner typically provides from across the entire operation.
There are of course limitations to how many users can be active, but that largely depends on what kind of users those are. Because there is obviously a big difference in terms of system load between a user who just watches the screen for alarms and all that, and a user that performs for example historical queries on the alarm history. The systems I have seen in operation typically have 10's of users concurrently active, up to even well over 100.
From a testing and validation point of view, I know the Skyline QA teams perform regression testing on new versions of DataMiner with 100 online users in the system (opening elements, services, views and requesting some historical alarms and time traces on select metrics). But again, the latter is a relative thing also, and this is referred to as "common user behavior".