Hi,
I don't think so there is a limit on user creations. Its all depends on contract between your client and SAP. every year you have to send the total number of active Dialog users to SAP. Based on the agreement and number of users exists in your system, SAP may go for contract renewal.
In the same USMM T.code you can see a tab called "Send to SAP" this will be used to send the user details to SAP upon request.
You may check below service market link for additional details.
https://websmp104.sap-ag.de/licenseauditing
Regards, Sai Krishna.