[keycloak-user] Unique Integer identifier in the Userinfo endpoint // automatically set up attributes?

Hello, for a specific application I need to provide a unique integer id in the information returned by the userinfo endpoint (openid connect). Using keycloak and mappers, I can return user attributes containing integers, but I failed to find how to automatically create those attributes in a way that would ensure each user would have an unique id (for example, first user would get 1, second would get 2, and so on). Is there a way to get such an unique integer id? (with either a mapper, or by automatically setting up some attributes at the account creation? Thanks, Rémy