I have 4 working phones (plus an old bar phone in a drawer).
My iPhone X, my primary phone on ATT Prepaid
My iPhone 7+, my backup phone/line on a Verizon MVNO. Generally used when on the road in case of emergency or in a poor stretch for ATT.
A Moto E4 that is my "home" phone. Average $3/month on an ATT MVNO. Give that number out when required, leave at home.
A Moto G5 Plus, truly a backup phone. No plan for it, but it could in a pinch be used for any of the other 3 phones while I decided what to do. For instance, if I broke my X, depending on the release cycle of the iPhone, I could either use it until I get a replacement for the X, or it could replace the 7+, and I use the 7+ until next year's iPhones came out.