Assuming it's built on level ground, how high should the walls of a raised bed garden be?

Depends on why you are using a raised bed.

If the raised beds are on totally unacceptable soil, concrete or asphalt, they should be 12-18" deep, or even deeper if possible depending on what you are growing.

If the raised beds are to delinate the planting areas vs the paths and contain the mulch in the beds, or simply provide an ordered look, they can be whatever height you want/have materials/inclination to do.

The really important part is that you have decent soil for growing that is a minimum of 12" and as much as 24", the more the better, depending onwhat you are growing. In most raised bed situations there is good soil that is deeper than the "raised" portion of the bed.

It really doesn't matter as long as you don't over or under fill with soil. It is mostly just looks and to keep weeds and grass out of the garden easily. Just remember it will drain faster than grounds level for the first year or so, may bed extra water.