I have read some bike shop owner's comments about how many thousands of dollars they had invested in bicycle tools. I think the key is that it's not too hard to get tools for your bike, but to get tools to work on any bike whatever is evidently a challenge.
For my Worksman cruiser, I've got:
1/2" and 9/16" box end wrench (fits axle nuts, seat post bolt, and handlebar bolts)
Second 1/2 combination wrench (for the other axle nut)
10 mm combination wrenches (fit chain tensioner)
8 or 9 mm combination wrench (fits coaster brake nut)
Phillips screwdriver (fits coaster brake screw)
Small channellock-type pliers, just in case
Plastic tire tools, which I don't normally use
I'm not positive on the sizes, but that's generally what I carry with me in my trunk bag. I've also got cone wrenches, but don't carry them with me because I don't anticipate overhauling a coaster brake on the road. For work here at the house, I've got regular socket sets, Crescent wrenches, etc.