Depends how far you plan to be from help, whether you will have cell phone reception, and what you know how to fix.

I would bring a pocket full of quick links for the chain, some cables, tubes and patch kit, a spoke wrench .... how much more would depend on where I was going and how long I planned to be gone. Definitely a set of tire irons. I recommend a small crescent wrench just in case.

Think what might break. Think what you might need to and be able to fix.

In my experience spokes can be an issue if you are very heavily loaded. You don't see a curb or pothole and "Twang!" There are rope spokes of some king ... otherwise you need to bring probably three different sizes and a cassette puller and chain whip. If you have a spoke wrench and are going to be near a shop in a day or two sometimes you can loosen the brakes and tighten the other spokes to get a wheel straight enough to limp along.