Using a smaller patch size is mainly useful for smaller detail representation. It should not hurt your larger features, in general, though they may be "negatively" affected by the smaller-scale features that show up when you reduce patch size. So basically the default of 20 is fine unless you're finding you're limited in how you're able to represent and/or act on smaller scale displacements.
As far as how many Compute nodes to use and where, as Ulco said, technically you should use one new one every time you modify the displacement *if* you want any subsequent displacement to be based correctly on the new displaced shape of the terrain. However in practice you can often use fewer Computes, and it is indeed desirable to do so to avoid high render times. There are a couple ways you can do this, for example combine your noise functions beforehand in color space, and find ways to get the shapes you want in a single displacement pass.
This can be especially effective if you break your workflow up into different scales, so the large scale displacement is created from a single displacement input that may be made up of the combination of several other shaders for example, but is only being displaced once. Same with the medium and then smaller scale displacements. This can allow you to get nice and complex shapes with overhangs and other unique and important features, without having to have a ton of compute nodes.
Sometimes it is not practical to do this and/or the result will be different from how it would be if you added one large displacement, then another on top of it, however subsequent large displacements can often cause overlapping geometry anyway, which is another problem that creates large render times and undesirable render artifacts.
So basically think creatively to try to create the shapes you want in as few steps as possible, especially when it comes to displacement. You can combine non-displacement data easier and "cheaper" than recalculating subsequent displacements with Compute nodes. So try to take that approach. And, as Ulco also said, when in doubt try adding a Compute node where you think it might make a difference and then see if you like the change or not.
- Oshyan
