Hi @LAURAMcCarthy ,
Welcome to Canvas!
When it comes to using free for teacher in your circumstance, I recommend that each of your teachers create their own accounts with their own courses, just so everything stays organized for them. However, do understand that since free for teacher does not provide you with access to admin-level permissions, you do not have the ability to view all the courses associated with your teachers unless you either create the courses yourself and add the teachers afterwards, or if the teachers add you to the courses. You also do not have access to change any theming, act as users, and enable or disable feature options at the account (admin) level, as those are managed by Instructure for FFT.
When it comes to courses, yes, the maximum limit for courses on Free for Teacher is 500MB. However, you are welcome to create as many courses as you would like to create, as well as enroll as many students as you would like to enroll.
However, if you are looking for admin-level permissions across an entire Canvas instance, you would need to request a demo for your organization and get information on pricing. However, to my knowledge, the minimum of account licenses you're able to purchase is around 200-300 users starting. However, I would see what sales would say, as it's always possible for things to change. That being said, since you only have 18 students and 4 instructors, I feel that Free for Teacher would be a great fit for you! And of course, if you have any questions, you're always more than welcome to come back to the community and ask! We're always happy to help you get started. I'll also provide a few links to some resources below to help you get started with using free for teacher.
https://www.instructure.com/canvas-free-for-teacher/orientation
https://www.instructure.com/canvas/free-for-teacher/account-comparison
All the best!
Noah