It asks you to chose “at least x” biomes from each tier; for example at least 1 biome tier 2, at least 3 biomes tier 3, at least 5 biomes tier 4, at least 2 biomes tier 5.
Size of the world doesnt seem to matter - for a larger world biomes will occupy larger areas if you chose the same number of biomes.
There doesn’t seem to be a limit for biome numbers - I didn’t see such info during the process, but maybe there is a pop-up message when you go for 50 biomes (just an example).
It’s up to us now to decide if we pick just the minimum number of biomes required or if we go for more. The more we chose the smaller the specific biomes will be I suppose (terrain will change faster when exploring such world).
There are biomes that have water in them - even pure water (sea) biomes. Other have rivers or pools of water in them. That also reminds me I forgot to mention before (I will update previous post) - before chosing biomes I was asked to decide what kind of river pattern I want on my planet - there were irregular rivers of different density to chose from as well as regular patterns of rivers (like straight lines across worlds or spirals like on some starting public worlds we know). There is also a choice of having no rivers at all.
Larger bodies of water are only accessible through chosing sea biomes.