You have an Email Program or Smart Campaign with an expected audience size, but when it is run, only a small number of leads are sent the email. For example, in an Email Program with 250 people in the audience, only 15 people were sent the email.
There are several things we need to check to determine why this happened. The easiest way to start is to create a separate Smart List with the exact filters as the program or campaign.
Add up the number of leads that may have been skipped for these various reasons and see if that accounts for the missing audience members. If it doesn't,
Check the Audit Trail to see if the Smart List has been changed between now and the time the emails were sent. If you still can't figure out why your email went out to fewer people than expected, open a support case and include the results of the above troubleshooting, and Marketo Support will investigate further.