To be honest, there's a lot of factors at play that can affect the answer: the wages the nurse is making, cost of living where she is, the standard of living the nurse and family is used to, how much money the nurse has saved up, how old the kids are, how much help--if any--she gets from the children's father(s), how much help she can get from family and friends when it comes to errands, childcare, hand-me-downs, etc. Neverminding the nurse's capability to mentally and emotionally handle being a single parent with a family.
I've known single mom nurses with one kid, and single mom nurses with four kids. Mom of one didn't necessarily have fewer struggles than mom of four did.
I would tell you to take it one kid at a time and see how things go.
I have two kids. I originally wanted seven, but it took a long time to get pregnant with #1, and once labor started the thought of possibly doing this six more times pretty much quashed that. Child #1 turned out to be on the spectrum so my hands were full for several years to even think about a possible child #2...who eventually did come along, and his birth involved a c-section and postpartum preeclampsia.
Thanks to modern medicine, there will be no child #3 unless I choose to adopt. I'm done.