These are very easy to add. On a typical Hogarthian harness, you'll find three: Two on the shoulder straps, and one on the left hip. Plus maybe a fourth on the crotch strap, if you need it. That's it.
To a lot of divers, this doesn't seem nearly enough. Especially since 'technical' stab jackets these days have so many D-rings you can almost dispense with a weight belt.
But it's actually plenty, because a Hogarthian rig relies on:
a) minimalist principles
b) drysuit pockets
When I was diving with a stab jacket, I had eight D-rings. Attached to them, I had a slate, two torches, a reel, a strobe, a console, an SPG, and an octopus.
Now, I have my main and backup torches clipped to my two chest D-rings, and (when I carry them) my big reel and dSMBi on my thigh D-ring. Everything else is pocketed or just not taken.
Lastly, there's the adjustability aspect. A stab jacket is not adjustable, so to ensure that you have D-rings where you want them, they put D-rings everywhere. But on a harness, you can put things exactly where you want them, so you have only as many D-rings as you actually need.
I did originally put a D-ring on my right hip, mainly for symmetry. I realised I didn't use it, so took it off again. Three D-rings is plenty for me.