The regs may say that (I can't remember, and don't have them to hand), but ....
If one sticks with the belief that the purpose of a sockets circuit's OPD is to protect the fixed wiring of the circuit (and not to protect {or 'limit' current to} anything downstream of the sockets), then I don't see what there should be any restriction regarding the OPD. In other words, provided only that the circuit's cable was of a CSA which was adequately protected by the MCB, I don't see why you could not have multiple 16A sockets on a 32A, 40A (or whatever) radial.
By analogy, as far as I am aware there are no corresponding restrictions as regards hard-wired (and 'unfused') loads - for example, I think that, theoretically, say, three 3kW immersions on a single 6mm² radial protected by a 40A MCB would be compliant, wouldn't it? If so, would three 16A sockets be materially different?
Kind Regards, John