Per this thread in the What new features would you like in 2.6.0? discussion, we should add an indication for the number of available messages in the response to the MSGS? query and HB response in addition to the ID of the first available message.

If there is more than one pending message, the additional number of messages should be appended to the response in a +n format. For example, if W4CAT has three pending messages for KJ4CTD, and the first message ID is 789, the response should be W4CAT: KJ4CTD YES MSG ID 789 +2. If 789 is the only pending message, there should be no "+n", the response should be as it is today, W4CAT: KJ4CTD YES MSG ID 789