Originally posted by Cooperal:

You can play on your own (1 player) and with up to 9 other people in a server to make 10 players.

I wouldn't recommend playing alongside strangers though. A playthrough takes a lot of commitment that untrusted people can easily ruin.

The world is more than big enough to accommodate other players who aren't necessarily friends of yours. There is plenty of land, biomes and bosses to go around. So I cannot see your playthrough being ruined, unless they actively set out to ruin yours.
No, the pertinent reason would be your willingness to host a server for players you do not know. Trust can be earned of course, but it's your extra time and work you'd have to invest in vetting them.
The option for 2-9 extra players to join you in a co-op venture is therefore aimed at you and friends of yours.
That said, the game can easily be played solo ;)