Airmail has two choices for 'Badge Icon Number'. The function of 'Badge' is - it fetches the number of unread messages which is used as a more preferred option and the second option is ‘count’, which would display a count of your all emails in inbox and not just unread mails. Also, you can choose either unread count or inbox count and by default Airmail has the unread count enabled for 'Badge'.
Whenever you receive new message from another sender Airmail will automatically update your dock’s Badge which is round red coloured like the one in the screenshot below:
The second badge count is shown on the left pane of Airmail which is termed as ‘Accounts badge’. It can also be set as unread count or all inbox count. When the badge count for particular account is shown in read, it implies unread count and if it is blue badge, it shows all mail count of that particular account.
The screenshot below shows when ‘Accounts badge’ is set for unread mails:
The screenshot below depicts when ‘Accounts badge’ option is set for all mails:
The Badges or Visual Balloons can be customised from Airmail Preferences > Appearance as shown below:
Please note that when you change the 'Badge' count for ‘Unread’ to just ‘Count’ it also makes changes to the Airmail Badge at the Dock.