Account types: standalone account, MCA and sub-accounts
Merchant Center has standalone accounts and multi-client accounts (MCA for short), with several sub-accounts underneath. Which type you have determines how often the switch has to be confirmed and how billing works.
Standalone account
One account, one shop, one confirmation, one plan. The usual case for a single online shop.
Multi-client account with sub-accounts
Each active sub-account counts on its own — both for the assignment to the CSS and for billing. The multi-client account itself isn't charged.
Agencies with many client accounts will find the right structure under White Label CSS. There, the assignment runs through your own CSS brand.