AITAH for disinviting someone to my friend's birthday that she didn't want after all?

A few of us couples are hosting a dinner at a nice restaurant for our mutual friends' 40th birthday. The husband and wife are turning 40 within a month of each other, and this seemed like a nice way to do things because otherwise we would all be celebrating them individually, so we decided to just rent a banquet room and invite 5 other couples...so that would be 20 people, as there are 5 couples hosting the event. The capcity of the room is 20.

We discussed this with the wife. In our group thread, and we all threw out names on who the other couples should be, we came up with 4. We obviously picked people that the husband and wife would want, but they are couples we all know. We still had space for a 5th couple. The wife said she was good with the existing 4 and would think of a 5th couple to invite.

By the next day, I had messaged the wives of two of the 4 couples because we were already on a thread about something PTA-related so I said to save the date for this hosted event.

Also by the next day, the wife of the couple having the 40th (the guests of honor) found out one of the 4 couples that we had thought of would be out of town and wouldn't be able to attend. So she texted us, "I'm going to add these three couples now." I said there was no space for 3 couples...there were the original 4, and one that would be traveling, so there is space now for 2 couples, to bring it to 5.

Then she says, "No, I am not really in touch anymore with (one of the couples I had already invited) so I can add 3 couples now." I said, "Oh we were talking about them in this thread, I though you wanted them." She said, "No, I'd rather have this new couple."

One of the couples who is coming texted me that the friend who I said to save the date to is excited to come, and I had them on the thread together so she was expecting it due to my message.

The birthday lady refused to change the venue we had all picked, and we can't have more than 20 in this banquet room.

I feel so badly for the univited person and also embarassed because I am afraid the univited couple and also the close friends of theirs (the invited couple) think we are trying to be cheap, like we could just change the venue, etc. The hostess had seen the thread suggesting her. Was I the AITAH for uninviting her?