There is probably some number of concurrent players that could use the public planets without having issues. All I can assume is the number is between what we have now and the 1000+ Steam and PS players we had for the Steam free weekend. Now is that total players? Probably not. But I would hope that it is a number that could successfully fund the game
As long as the growth is at a moderate rate, the developers can always add more planets. If we got to 50k + players I wonder how many planets that would actually require? Several hundred at least I would think. I do think this size universe makes the game very different from what it is now. As an example, is there really one guild of players that could manage a portal network across so many planets?