Messages Sent: No. of messages sent in a campaign. This number may or may not match the number of contacts in your campaign list. But it will never exceed the number of contacts in your campaign list.
Delivered Rate: A percentage value of the number of Messages Delivered out of the number of Messages Sent in a campaign. The messages delivered number is provided by Meta.
Read Rate: A percentage value of the number of Messages Read out of the number of Messages Sent in a campaign. The messages read number is provided by Meta.