Like others said, there a currently 5 Bosses with more to come. But actually, the game ends when you want it to end. Depends on your goals.
-Kill all Bosses
- Build large Castles
- Explore
- all the above.

The devs have said there will be an actual end to the game when it is finished. That probably won't be for at least a year.

It ends when your patience runs out. The entire game is fuelled by that, and only that.

The devs have recently hinted at the possibility of a final zone or dungeon opening up once all of the world bosses are dead, but this is a generous interpretation of what they've stated so who knows.

Well the objective is to kill all the bosses but the game isn't finished yet so we don't know what happens once you do so.

I guess a true ending is once your build yourself a 1.st very large castle.

I think finishing the bosses will be the end of the game for those who just want to do that.

I doubt they will remove you from the world you built up, that would seem like it would upset builders and such.

But yeah there will probably be some song and dance at the end <or scrolling text saying thanks from Odin or whatever>

The game seems like it has a solid vision for where it is going so far, so I would expect some kind of end of some kind. But I doubt that will keep you from continuing if you want.

Actually at the moment there five world bosses. I've only two down on my server and this by ~680h playtime. Other people are playing faster than me, they kill the bosses in a speedrun and they cry there is not enough content and they get bored...
I have a lot of fun to collect ressources, to build buildings and I have time to kill the other bosses.
A real ending isn't there at this time, because the full content ( (I mean) five bosses more) are missing at the moment.
Further a few bioms aren't ready, you can get inside, but there are no enemys inside.
I don't know if Odin take you with him, when the last boss fell and the game ends here, or if there is an open end. We will see.