In how many different system a user can login using single Userid? Is there any

A user can login using the same user id so long as every system has the same user account. If you are talking about single sign-on options, you will need to make a custom plugin to integrate Appian with another single sign-on service.

No need for a custom plugin. Utilize SAML
forum.appian.com/.../Authentication.html
"SAML is a set of standards that govern communication between a service provider (in this case Appian), a client, and an identity provider. The standards allow for secure exchange of authentication information over multiple domains and environments."