mil inviting her friends

My FH and I have made a list of who we want to come to the wedding. We have to keep it to 100 people but my FMIL wants to invite several extra friends. She was a single parent growing up and says these people were huge in my FH life when growing up but he and I have been together 7 years and have never met them in my life. I’m having a tough time telling her no because she is financially helping a lot with the wedding but I honestly don’t want to be paying for more people then I need to. Also she’s the type of person to say if the limit is 100 and there’s 90 people RSVPed she will think there’s 10 spaces she can fill. I’ve tried to be stern with her and my fiancé is no help at all when it comes to this as he just tells me to tell her no but chokes when the topic comes up. He is an only child so I do realize this is a big deal for his mom and I do want to be sensitive to her wishes but I’ve already included many people for her already I just don’t want to continue adding. I’m a huge people pleaser and I’m really trying to break out of my shell by saying no to things she wants that I don’t want. Sometimes I feel like she thinks it’s her wedding not ours. How do I approach this???