Silo is an on-demand, remote, disposable browsing environment which executes browser code without taxing or exposing local resources. The User Limits feature allows admins to set hard limits on the number of users that may be loaded into an org or tree of orgs. This lets admins controls how many users sub-admins may set for their orgs and makes it less likely that a customer may go over their contracted user amounts.
From the admin console navigate to the highest level org you would like to control. Press the Policy Panel button and select User Limits Edit
Next, select the max users for the two fields and press save:
Next navigate downward and again from the User Limits page select the limit for the sub-org's users and press save:
Once you have completed configuring your orgs, you may go to the top level org and view the status of all of the User Lmits: