Hi Sumeet,
I'm not aware of a hard limit on users. There may, however, start to become manageability issues when using UIM managed user accounts vs integrating with a LDAP backend.
There might need to be some additional explanation based on your question about the UMP console. In UIM, there are two classes of user accounts; "Real Nimbus Users" and Account Contact users.
Real Nimbus Users are the accounts that are managed in Infrastructure Manager. These accounts have no restrictions based on origins, although you can restrict their overall capabilities through ACLs. Again, they'll see all devices, all QOS and all alarms across all origins, within their ACL permissions to access those particular sections.
Account Contact Users are effectively "web only" users. They can only log into UMP and will never be able to access Infrastructure Manager. They too can have ACLs placed on them but are also limited by the data they can see based on origin of that data.
Regardless of whether you are using Real Nimbus Users or Account Contact Users, you do not need to set up logins for these users in UMP as a separate process. They will be created automatically upon their first login.