Fiancé’s mom trying to control guest #

I would suggest you and your partner sit down with FMIL to show her the price per person with taxes and service charges, and make sure she can accomodate this. Most vendors like venue have to be paid in full before the event, so there is no room for error. If she cannot afford this charge, and you don't really care about extra persons, then your backup plan would be suitable. Reinforce that it is awkward to invite persons to the wedding who will not receive a formal invitation in the mail. They will also not have a seat for dinner. Good luck.

Its absolutely not unreasonable to put your foot down. But you and your fiance need to sit down with her together (as it's his mom) and let her know that the guest list is already finalized, she cannot add additional people regardless if she wants to cover them or not, let her know that you've selected your nearest and dearest and would like to keep it at that list and no one else. If you need to find a compromise, suggest her hosting a dinner with those people after your honeymoon, but they are not invited to the wedding and will not have a seat.

When you say she invited do mean verbally or she actually sent formal invites to them? She can tell whoever she wants to that they are invited but unless they get an official invitation from you and your partner there will not be a space for them. I would have a chat with your partner and decide the absolute final head count and then your partner needs to tell the mom we have decided for numerous reason not just financial that the final head count is X and we can not invite anyone else. She will need to apologize to the guests she invited verbally and let them know due to space constraints the wedding is being kept small. So frustrating you have to deal this, hope it goes well.

"We won't be able to accommodate your guests at our wedding". Don't give reasons or excuses, or she'll find a way to "fix" it, ie offer money. Have your FI deal with her, it's his mother, but get on the same page and don't give an inch.

You almost seem to invite the idea of her contributing the extra money for the additional guests. Can you add additional guests without upsetting the balance you have with the current guests (like not insulting people you would wonder why "those" people were included and not them)? If she paid right away, it might resolve a few issues. But that may not be the real issue and, as some have said, both of you, as a couple, need to make sure this will not be a pattern after you are married.