I would like to be able to approximately 120 students to share comments with each other in my project, but I don’t want them to share a link with their friends, so I’m thinking I should change the Share option to ‘only people invited to this file can access’, but don’t know if I’m limited on how many I can invite.
There was never a limit. The limit that got raised is for simultaneous connections in the multiplayer (now up to 500), not for people invited to the file.
Note that if you are gonna invite them by email, you may be temporarily shadow-banned if you send too many invites. If that happens, the restriction should be gone in 24 hours and you will be able to invite more people again.