This interview question is intended to know how much experience he/she has got in managing users. It is mainly asked for Salesforce Administrators.

Salesforce.com supports thousands of users.

As
the number of users in a Salesforce.com organization is more, it results in more
number of Roles, Profiles, Sharing settings, Reports and Dashboards, Permission Sets, etc… So they can easily judge whether he/she will be able to face complex security controls.

Hence
if a person had worked as a system admin for an organization containing 10,000+
users, his experience level should be reasonably high.