Hi Thorsten!
WiFi radios nowadays support anywhere between 8 to 16 SSIDs, but in practice you would use only up to 4 SSIDs or less. That's because the AP has to transmit beacons for each SSID which consumes wireless bandwidth.
Instead of multiple SSIDs you may try per-PSK WiFi VLANs. Basically, you have one SSID and give each client (or group of clients) a unique PSK. The AP would then map each PSK and its corresponding WiFi traffic to a specific VLAN.