I use Dashes to make it easier to read.

  1. Arch-Linux
  2. Cent-OS
  3. Debian
  4. Fedora
  5. Kali-Linux
  6. Knoppix
  7. Linux-Mint
  8. Mageia
  9. Open-Suse
  10. Puppy-Linux
  11. RedHat
  12. Slackware
  13. Ubuntu

Obviously there's more. But you need to stick with the major families, and only one or two levels deep. You can try as many Linux (and BSD) Distros as you want. But focus on the ones the major tech/web companies use; and what is used in the enterprise - because that's what you'll be working on every day. Also, the more-obscure Linux Distros tend to come and go, have hardware & software issues, and just generally take your time away from the MAJORLY-SUPPORTED Distros - most of which are listed above.

Also, this is not DistroWatch, nor Wikipedia, so you are welcome to delve further into this topic.

SUMMARY: Spend your time mastering the MAIN Linux Distros. Don't invest too much time into the lesser-used, frequently unsupported Distros (that come & go). But, by all means, try as many as you feel like. And even explore the educational path of Creating Your Own Linux Distro.