In very approximate order, and I suspect that there are (wild approximation) half as many users for each step down the list.
One estimate I've seen is that there are ~7K users of openBSD, in total, worldwide.
There are a few distros of FreeBSD. None of the others have distros, AFAIK. PC-BSD was a FreeBSD distro. iXsystems acquired it, turned it into TrueOS, and then killed it 4Y ago.
Having had some experience with FreeBSD, I'd pick BSD over Linux any day, the primary reason being its unique consistency, which is after a few days a breeze of fresh air even to a newbie. The vast majority of software that runs on Linux is available for BSD as well. Sadly, hardware support is not that great.