Responses
No there is more royal, it is just only a couple of them could be able to impregnate the uke because the other weren’t at the right age (maybe too young). Go back to chapter1, I think they mentioned 3 name of royal at the right age, not including the prince because uke and him at odd. Then assuming your theories correct the murderer impregnated uke will mean that he has royal blood.
Wait, if only the royals can impregnate men, and there's only two royals (the white hair and the considerate Duke earlier), then how did the serial killer impregnate the ML, assuming it's the serial killer and the serial killer is neither of the two royals?