On bikes with clincher tires I carry one spare tube and a patch kit. A patch kit can handle as many flats as it has patches, whereas a spare tube can cover just one. Also a patch kit is lighter than another tube. For years in the old days I didn't carry a spare tube at all. But a spare tube has its advantages if you can't find the puncture.

Sometimes I replace the tube, sometimes I just patch the one that flatted. It depends on how quickly I can find the puncture and what the prevailing weather and environment are. Patching a flat means I still have the convenience of a good spare tube if it becomes necessary. Just replacing the tube means on the next flat I won't have the option. I have encountered multiple flats occasionally. I've also seen other riders swap in a spare tube, puff it up with a CO2 cartridge, and then notice the tube pinched between tire and rim and watch it go flat immediately. That sometimes ends up with me giving them a tube and letting them borrow my frame pump. Or I'll swap in the tube and pump it myself.

On bikes with sew-up tires I carry two spares on most rides, sometimes just one on my commute and then if I have a flat I worry really hard until I get within walking distance of home.