Both of them is about customization. Hard. I don't think there's two similar Slack or gentoo installation. So, for me, at least, it feels there's really no need distributions of those. Maybe it's just by definition, though.
I think it's somewhere around 15 derivates for Gentoo. 12
It's about the same number as lots of other distros?! I don't see any substancial difference there. Except when you compare it to something like Debian. There are some hugely popular distros.
People make new distros based on existing distros to alter the software stack in the original distro. Gentoo allows you to use essentially whatever software stack you feel like, it's less of a traditional distro and more of a general-purpose Linux package manager and dependency resolver. It's not very opinionated at all.