It's a hard question to answer, since it depends upon where you're riding, how far from civilization/your truck, maintenance abilities, etc..
For me, I ride singletrack that is typically 25 to 35 miles loops. The trails tend to cross many backroads/jeep trails so I could shorten a route in an emergency. Over the years, I have reduced what I carry and now carry the basics or bare minimum (still more than my buddies carry).
I used to use a fanny pack, but I don't like the weight on my hips and banging around. I use a camelback with extra storage capabilities. I bought it at Costco (I think the pack is called High Sierra)... anyways it looks like the camelback Mule version. I carry my water, some granol bars, my tool wrap, tow strap, car keys, cell phone, photocopy of license and insurance info, and $20 (for emergencies).
In my tool wrap, I have the basic necessities to fix most things on the trail. This covers most of the minor repairs and gets me back home.
- 8, 10, 12, 13, 14, 15mm combo wrenches (these have a different size on each open end). You give up the box end but you minimize the number of wrenches.
- T-handle with similar size sockets including 6 and 7 mm (for carb & hose clamps).
- Multi-purpose tool (like Leatherman) which has pliers, sidecut, knife, etc.
- Folding tool with allen wrenches (4, 5, 6mm) and screwdrivers (straight and phillips). Tusk makes a nice cheap one (from RockyMountain ATV/MC).
- Miscellaneous items like zip-ties, master link, and section of fuel hose (to transfer gas in emergencies).
I do not carry tire irons or tube, unless it is a "long distance" trip. Most trail rides, I can ride back with a flat tire, zip-tied to the rim in emergencies. So, I typically don't carry them.
Hope this helps!!!