Four of five indoors are pretty purr-easy. They will up the volume if ears scratched, but they will purr at any attention or even a look.
The exception (naturally) is HRH Rosalind. She has a great purr, but she does not bargain. She only purrs when she chooses to. Now, her occasions for choosing to do almost exclusively involve being on me. But I cannot “make” her purr. Her choice to put the switch off or on, and the paw on that switch is hers, not mine.
I have never in my life heard her purr in any proximity to anybody else. Rosalind, I think, really would be quite happy if the rest of the human and feline races would all go jump in the lake and leave the two of us alone in the world. Her idea of paradise. Others of whatever ilk are only disruptive and don’t add much.