Especially since starting out you won't have the entire surface to build on, you have the surface only so far as the corruption is to the east and west. Because players starting out can't cross the corruption easily(at all). And even for mid gear folks, it's still a tough obstacle between their house and respawn every time they die.
East, west, up, down... the corruption can be dug under or built over with sufficient effort. Just need a lot of non-dirt blocks for the latter and patience for the former. I've been seeing screenshots of underground houses all over the place, now, and even a few snaps of the sky islands.
Can build = true is probably a greater weapon than everything short of teleportation and wall hax ala bosses and fire imp projectiles, and it's a weapon available right from the start. Even devourers and skeleton worms can only jump so high, heh.
As for the numbers thing... I'unno. It'd probably depend more on how much the players clumped together than anything else. Everything we've seen says these maps are freaking
huge. Easily enough for 8-15 folks to have their spot for mining and building, especially if some of them's specifically grouping up to work an area over.
The spawn thing's an issue, though. Looking forward to support for moving the team spawns around. That'd at least help the problem a bit.
EDIT: Linked on SA, from PBat's latest.
The maps are large, people. Space is going to have a hard time being an issue
Remember. They're wider than they are tall.