Under CSP, you can add or reduce the number of licenses any time. You are billed with daily proration.
You are required to have at least 1 license QTY of the subscription you have purchased of a 12 month term, otherwise you need to suspend the subscription. Currently there is not penalty for the suspension of services under CSP.
Depending on the Microsoft Online Service, the customer will have limited or no access during this time unless you have purchased licenses through other licensing channels.