count

COUNT  -  count messages

Usage:  COUNT [message-sequence]
  Unless specified, the message-sequence is "all".

COUNT displays the message numbers and number of messages in the message 
  sequence.

Examples:
    MM>count                           #how many messages, total
    MM>count unseen                    #how many unseen messages
    MM>count from lisa since Monday    #how many...

For more help, type "help" and one of these topics:
  message-sequence   information