on a T300 AP where the max number of concurrent users connected is 500 (based on datasheet) i created 4 different SSIDs. I know that max concurrent users is related to the encryption and authentication methods. Let's say that the 3 SSIDs of them are open BUT the forth one has both WPA2 and 802.11r/802.11k enabled. The question is if the max concurrent users on the 3 open SSIDs will be affected by the 4th SSID.
You see, the 500 max user number is a theoretical value based on the maximum number of associations (AIDs) the hardware could technically handle depending on things like authentication methods. How many client devices you will actually be able to serve in real-life is an entirely different thing and I can assure you it will be nowhere near 500.
The reason for this is that, regardless of what the spec sheet may show, the limiting factor in this case will be available airtime not the hardware. Simply put, there will not be enough airtime available to serve 500 devices with just two channels (2.4 GHz and 5 GHz).
The actual maximum number of concurrent client devices you can serve reliably will depend on: 1) client device mix, 2) applications you need to support, and 3) RF environment. Here are two examples:
Client: MacBook Pros with 3x3 radios
Application: Email and browsing (low throughput)
2.4/5GHz split: 30/70
In this case you could do a little more than 150 clients per AP, assuming little CCI from neighboring cells.
Client: Smartphones with 1x1 radios
Application: Voice, low latency
2.4/5GHz split: 30/70
In this case all you will probably be able to support is about 20-25 concurrent client devices per AP, assuming little CCI from neighboring APs.
Now to your question about the impact of an 11r/k SSID, that in and on itself should not reduce the number of client devices you will be able to serve. What does have a negative impact is the number of SSIDs per AP. You see, each SSID behaves as a virtual AP which broadcasts beacons, responds to probe requests, and handles other management and control traffic. So the more SSIDs you have configured the more overhead you are introducing in the channel and hence the less airtime that will be available for communications with client devices.