I do not believe it is possible to automate this.
When you send out the meeting invite, you could have a statement that only the first 50 will be able to attend. Then when people “accept” the invitation, you also get a response. Once you see you have 50, you can respond that that their request was not within the first 50, but they are on a waiting list. You would them remove them from the meeting. Since you have a copy of the emails, you have the sequence of the responses.