The API limit will be applied at the BU account level in Site24x7, and not based on the units within a BU account. The API Calls Usage per Day shown under every unit in the Site24x7 web client (Admin > Developer > API) will be the global value for that BU account, and not for that particular unit.

For example, if a user signs up for a PRO plan, then the total API calls allocated for that account will be 10,600 (53x200) per day. This is irrespective of the monitor usage by the units in that account.