Gmail doesn’t display a true message count by default, because Gmail uses the conversation view. Turn the conversation view off to see the real number of individual messages in your Gmail account
It’s sometimes useful to know how many messages you have from a certain sender or on a particular date. Since Gmail uses the conversation view by default, you only see the number of conversations. When you turn conversation view off, you’ll see the individual message count for your inbox, sent mail, labels, and in search results. You’ll see something like, “1-50 of 2139”, in the upper right-hand corner of your message list.
Find the Number of Gmail Messages [Google Operating System Blog]