Placing a mailbox on retention hold suspends the processing of the retention policy for that mailbox. Retention hold is designed for mailboxes whose users are on vacation or away temporarily. During retention hold, users can log on to their mailbox and change or delete items.
M365 Manager Plus' bulk mailbox management feature allows you to configure retention policy and retention hold period for multiple mailboxes in one go. To apply a retention policy, make sure you have a well drafted retention plan, and created the same in Microsoft 365's Exchange Admin Center (EAC)
In M365 Manager Plus, there is no restriction on the number of mailboxes that can be managed at a time. read more about the management features of M365 Manager Plus, here.