Using older versions of Outlook for Mac for personal email and calendar
It may be because of the very large amount of messages you have in your inbox/folder (I am guessing 1,000+); it is only showing messages that are loaded. When you scroll to the bottom of your list, if you see "Load more conversations" you know that it isn't a full count. If you continue scrolling it will eventually load all messages then you can select all to get a full count. But be patient, it may take a few seconds to select them all. (True confession: I have well over 2,500 messages in my inbox. Don't judge me.) Personally, knowing the complete count isn't important for me; my boss keeps her inbox count at almost nothing, she requested I find an answer.