Currently you need to make two calls to get the total count of unread messages or to get the count of unread messages from a single channel (/users/me/teams/{{team_id}}/channels and /users/me/teams/{{team_id}}/channels/members).

To avoid the first request, it would be nice to have a new property e.g. new_msg_count that returns the current new message count for a channel. This is currently only possible by substracting the msg_count (second request) from the total_msg_count (first request).

This topic has already been discussed on github: https://github.com/mattermost/mattermost-server/issues/8591#issuecomment-383909505