Add transaction to reopened batch
Its a same procedure to assign transaction to reopen batch as of open batch by visiting the transactions list page of batch and then selecting and assigning transactions to batch. The number of items or total amount in batch doesn't matter and it does get updated automatically once a batch is closed. But some organization prefer to have exact count so that it matches with their accounting integration with Quickbooks or Great plains etc

Ref: https://docs.civicrm.org/user/en/latest/contributions/accounting-integration/ 

HTH

Pradeep