There is no any checking for number of concurrent users against licenses for catalog product since 14.1 and 17.1 “ is correct.
Catalog doesn't check catalog licensing by usage ( i.e. how many concurrent users ) .
If your license is 5500, it doesn’t mean you can only have max 5500 concurrent users to access their catalog system . Instead , technically you can have over 5500 access to your catalog system as soon as you catalog app ( and its computer resource ) can handle it .
If you buy one of the Service Management packages (SDM, APM, or ITAM), you get charged/measured based upon the SDM or APM/ITAM licensing and get unlimited use of Catalog.