I couldn't think of a less dramatic way to word the title. Maybe "broken hearts" is a bit strong, but are there many of us living our lives hankering after lost loves? Even that sounds a bit twee!
Me personally - I've been in love with someone for about 10 years now. We had a brief relationship, it didn't work out, we both moved on. Except I haven't completely. I think about him every day. I know that I will never feel this powerfully about someone else. I imagine he doesn't give me a second thought. It doesn't stop me living my life, and I try not to give it too much headspace. But it's always low-level there.
I sometimes see posters saying similar on here on threads about things we'd never tell other people. And I have a couple of friends who feel similarly.
I know no one has an answer for how many people feel this way - just wondering.