Any input on what percentage of people encounter a swarm?
Its likely you will not find an answer to this question unless you narrow it down geographically. The actual answer to your question though if I had to guess is likely much less than 20%, could be as little as 5%.
What to do?
Enjoy it in all its glory because you may never see one again unless of course you are a beek or bee removal specialist in an area with lots & lots of feral & domestic colonies, somewhere likely in one of the southern states like my own.
Swarms are beautiful to behold, quite noisy, not that different than some sort of electrical buzzing sound (no pun intended), usually very gentle, as long as you don't swat at them, which you should remind people not to do.
It all happens very quickly, as they pass overhead.
Best advice I can think of unless you are a beek or wannabe beek trying to catch the swarm is to simply take it all in. That's right, tell them they are not to be feared and to enjoy the experience!