This is an easy one, but you need to know that 1) the option is there and 2) where to look.
Go to the web-interface to zoom at your institution, and click on log in:
Now that you're in, go to to Reports:
Then go to Usage:
Now, search for the time period that you're interested in:
And click on the number of attendees. Note that if someone gets disconnected, and then reconnect, they're counted twice, so this is not the value you want to use:
Instead, make sure to click "Show unique users" to see how many students actually attended. You can also export it as a csv file which is probably a good idea since you'll need to count by hand otherwise:
That's it.