When you sign in, You can see tiny site stats chart(displayed in above screenshot) in your toolbar called a Sparkline.
The number in Sparkline represents the highest number of visitors visited in 48 hours of day.
You can find more information in wordpress.com Stats Support.
I have copy pasted the content below from this link so that it will not affect if that link ever changes its content in future.
You can also access your Site Stats page via the tiny stats chart in
your toolbar called a Sparkline. The chart consists of 48 upright bars
representing the last 48 hours of page views for the current blog,
with darker lines indicating nighttime page views. Note that the
toolbar will only appear when you’re logged in to WordPress.com.