We've had 1 broken leaf spring. We've been traveling fairly extensively for 15yrs. As a kid, my Dad's camper had 100,000miles with no broken leaf springs.
The one that did break, it took out the adjacent tire, so it got replaced on the side of the road. Because it shredded the side of the tire, the tire wound up wound around the wheel hub and to get a guy out who had cut off saws on his truck to remove the tire. By pure chance, he had a spring on the truck of the right size from another job.
After we got back from the weekend trip, we had a guy replace all the springs proactively while sitting in our permanent site. While we were at it, had the axles flipped for more clearance, since he had it torn apart anyway. Took a couple hours.
It wouldn't hurt to keep a spare but unless you have an unusually heavy duty axles or are going very remote, the springs are pretty standard.