After choosing segments in a campaign, a number of contacts will be added.
Sometimes the number of emails sent may be less than the number of contacts added in the campaign.
If so, there are several reasons:
contacts that were already bounces in the segment before the shoot (it means they are in "do not contact")
contacts that have unsubscribe before the shoot (it means they are in "do not contact")
it is a marketing email and contacts have already received it (marketing email can only be sent one time to a contact)