Having a lot of seams is fine as long they’re not anywhere visible. The seams in that head are in the back of the head (which will be covered by hair) and between the head and the ears i think, and the density is about the same, so it almost doesn’t matter.
Seams should be placed where they will be the least visually jarring, or barring that, at least where there’s a hard edge (smoothing group difference.)
There is a normal tension between “as few connected patches of UVs as possible” and “as little stretching as possible.”
I’d say that up to 20 separate patches is probably OK; more than that, and you have to start looking for ways of merging patches without stretching the UV geometry too much.
Separately, the borders between the patches need to be bigger the further away your MIP map is. If you create a texture at 4096 resolution, but on the screen it’s only 500 pixels high, you’ll be down in the third or fourth MIP level, and thus your borders need to be 8-16 pixels at the 4096 resolution.
That, in turn, has tension with the “use all of the texture space” suggestion. If the color and normals of the different areas are “close enough” then you can put the patches close enough to each other, because the MIP map filtering won’t cause horrible artifacts when they blend.
The texture above seems to have about 10 patches, which is not too many, and it seems to put areas that are very similar in color (face, neck) close to each other, which solves for the MIP bleeding problem.
So, AFAICT, that looks good!
Generally it’s more about using a balanced number of seams, too many and you are wasting texture resolution for padding which isn’t efficient. Too few and you get too much stretching or issues with sharp angles. It’s even more complicated when you have to worry about lightmap baking as well.
Maximize texture resolution on areas of interest, i.e: the face.
Place seams on strategic regions, i.e: behind the ears.
Make as less seams as possible.
Reduce shape distortion as much as possible.
Make sure your normal maps blends at seams regions (this one is hard)
Last one, normal map seam blending, sometimes can be solved with xNormal plugin for Photoshop; but sometimes you gonna have to paint seams manually or blur them, or your model will have hard edges everywhere, in the game engine.
In case I cause some confusion, I was talking about removing normal seams by hand when tools won’t help;
Somes times one has to flatten the normals on locations with 128,128,255 color to remove UV seams: