Headcount: How to count blank cells with corresponding data in same row?

Hello!

Say I have a table where I am signing in visiters with a TIME IN and TIME OUT columns. I would like to count those who have been signed in but not yet signed out. I want this to be automatic so my employees can know the active headcount without having to know how to use function or count manually.

Example:
Lets just say I would have preset 500 rows on my table and by end of day I signed in 200 people.
Who out of the 500 rows has been signed in but not signed out yet?