For a longer list, split it into several campaigns within your plan’s limit per campaign; on Business the monthly limit counts the calendar month in the time zone of your settings (UTC if none is set). Addresses that bounced hard, opted out of your company’s messages (also in the CRM) or whose owner asked for erasure are skipped.
Campaigns are sent through your own mailbox, so your mail provider’s sending limits apply as well, and they may be lower. If many emails bounce or are rejected, the Deliverability Guardian pauses bulk sending until you clean the list.