Patches come in two sizes. The size most shops stock is too big for road bike tubes; they're wider than the deflated tube when the tube is flattened for repair. The only problem I've had came from not being able to get the whole patch to stick to the tube. Smaller patches work fine.

The tube in my back tire is on its third tire (due to two terminal tire sidewall cuts). It's got five patches on it so far.

I carry two spare tubes...because I've needed two before and only had one. One is new, the other is patched and tested.