You can have up to 34 (max number of regions for a 3km world), but it is recommended you only pick 12. Which the combination of numbers and such depend on the tier you choose.
@Stretchious covered the requirements here: FAQ: Sovereign and Creative World
So, if every new sovereign has a chance to generate with a new color, and then everyone else gets access to that color…
What happens if a sovereign planet doesn’t get renewed? Does that color stop being available for other planets, or is it unlocked forever?
Unlocked forever AFAIK
Once a color is unlocked, is it available for selection on any private planet? So like if someone unlocks black gleam on a T6, can someone generate a T1 planet with black gleam? Or will there be color restrictions based on type/tier/biome/something else?
Yes, & gleam is a block that can appear on any planet tier.
There are resources that have tier requirements (gems, etc)
So… after checking out, where do I go to customize?
You select the terrain during the purchase process (right after via email - look for the “configure your world” link .) You don’t need to copy the key or anything. It’s automatically filled in the form for you.
You select the colors once you’re on your planet and place the world control.
Ah dammit.
After purchase/checkout you will get an email with a link to go to the biome customisation form.
So if I purchase a world on the website how will I be able to access it from PS4? Gleam Club is purchased from the PS4 store.
You access the world via in-game menu (places > worlds)
— I don’t remember if you have to warp to them 1st or not though.
The in-game shop menu has a new section to enter the extension keys for your planet, once you’ve bought them on the website.
When you purchase via the website, it creates a code. Then when you follow the emailed link and customise your world, that code is prefilled - you then enter your account and character names which then ties the world to your Boundless account.
Do we get an email when our planets are done?
no email 
Does it just appear in worlds
how will u know if your world is ready?
So do we get to customize colors with Soveriegns?
Yes. Once you’re on your planet, using the world control.
Cool I assume this includes naming the planet too? Please say it’s so 