Theoretically there is no limit to how big a portal network can become outside of the amount of portal fuel you can acquire to fuel the network. So even if there’s 3000 worlds, you could, in theory, link all of them together with a single portal network as long as you have the portal fuel to allow for it.
I suspect there’s going to be multiple portal hubs and networks working together that basically link to one another that link to every public world in the game. 
Good lord. I get lost already. o_O Oh well,the more the merrier!
I believe @James has said many times that if you use the public rules for your private planet then the planet can connect to the general servers. So, you couldn’t just pick rare items to be there on a starter planet. However, if you want to use private rules rather than the general rules then yes, you could have rare things, however your planet could not connect to the general public servers and you could not take your private resources to the general servers. It’s your private planet with no access to moving things to the public servers. In my mind I think of the private worlds with their own rules as being like the test servers. You can’t farm on the test server and go sell the stuff or use it on the public server.