How do you go into each relay and get that info and how do you know it’s unique npubs? I have a hard time believing we have 42 million unique npubs on NOSTR. If so then yay however it seems improbable based on the biggest users and their followers and the number of daily posts we have here ?

Anyone can generate public keys and there is a lot of spammers. The stats website says: 1,228,201 users 108,384 trusted users It will never be possible to truly get accurate number of real users on Nostr, for multiple reasons.