I assume A is a person who is saying the first line, and B is a person who is saying the second line. I've changed it somewhat below:
Andrea: "I would love to live in beautiful Tuscany."
Bobby: "Wouldn't we all?"
Notice that Andrea is using the modal verb "would," and Bobby responds also with the modal "would not." Bobby cannot say Wouldn't we all? unless Andrea has used "would."
Wouldn't we all? is a set phrase that is short for, Wouldn't we all love to live in Tuscany? You can translate it to mean, Everyone would love to live in Tuscany. It's a way of being agreeable with Andrea.