hmmm here is a free book. it has about 40+ sutta of a stream enterer (none-contradict each other whatsoever. We buddhists want to know if we have achieved this level of dharma.
Buddha allowed us to proclaim ourselves so and here is the tool.
I remember a sutta from pali cannon where this rich merchant invited buddha to receive meal at his mansion in a week. later a king or prince desperately wanted buddha to come eat at his place on the same day (i dont recall why). Buddha said he couldnt change the reservation without consent from this merchant because he invited him first. So buddha sent ven Moggallana to this merchant to see what he would think about moving the reservation to a week later. the marchant would agree if ven Moggallana could confirm 4 (or maybe 5) things 1) would he still be alive ? answer was yes. 2) would buddha still be alive? yes. 3) would he still have his wealth? yes. 4) would he still have the unshakable confidence in buddha? ven Moggallana did not answer the last question citing that this was something the merchant had the ability to answer that himself. finally he agreed to move the reservation to a week later. Point is, a confidence in buddha, dharma, sangha is one characteristic of a stream enterer. And Buddha was explicit that only him and yourself can make such a claim.
How common is it? Buddha said in one sutta, plenty. not 10, 100, 1000+ as long as 8 fold path is practiced, world is not going to be absent of an arayan (those who is a stream enterer or higher. perfectly good word ruined by Nazi). also compare to another sutta, he compared dust on his finger tips to the entire earth as reference to numbers of Arayans compare to all other beings (seen and unseen) so the ratio would be large!
read the book and ask yourself. only you can proclaim yourself ..