Why wasn't the campaign sent to everyone on the list(s) ?
There are TWO different reasons :
All the contacts that have previously unsubscribed from your marketing campaigns, will not be sent a campaign even if they are apart of a marketable list
AND / OR
If a contact is part of multiple lists, they will only be sent a campaign once
How can I determine how many contacts will receive the campaign?
You will be able to see exactly how many recipients will be sent the campaign by viewing the summary provided at the end of your campaign
by going to the sent section under campaigns and viewing how many recipients were sent the campaign.