<<But I can't get information on how many guests came into the class with that code. Is what I'm looking for even possible?>>
Since there is one and only one guest account, all these people are exactly the same as far as Moodle is concerned: one anonymous guest accessing the course many times.
Guests don't use the enrollment keys because enrollment keys only pertain to proper enrollment of normal users using self-enrollment. Once used and enrolled, the enrollment key is not needed again by that user.
The password you set for Guest access on a course is separate from that (and should be different.)
The way to get information you want is from the site logs. You could search the logs for that specific course for guest access and then by counting the number of unique IP addresses, you will have a rough number of distinct accesses to that course. (Provided of course you are logging guest access, which you should be). If your site is using something like Google Analytics, that will give you similar data.