The CodeHS instructions may not be adequate to let you know what to do here. You might figure it out or find an example online, but there's a simple way to do it.
At the top of your function, make a count variable. Set it equal to 0.
count = 0
In the for loop that you make later, have that count increase by 1 each time. There are two ways to do this:
count = count + 1
count += 1
The second option does the same thing as the first - it's just a cleaner, shorter way to code it.
After your for loop finishes, have it return the count.
return count