I do not know how I can enter an user with multiple clients
Do you mean Virtual Sub clients of the same client?
This can be done under client settings. Simply put what you would like it to be called and you will see a virtual client populate on the status page a few moments later if the original client is online.
I think you mean to have one user manager multiple clients like this https://forums.urbackup.org/t/how-to-set-one-user-to-manager-several-clients/2818/6
Thank you, the link to the previous discussion was what I needed