What do you think will be the highest ever achievable elo rating?

Elo ratings don't indicate a fixed level of skill, but are to some extent dependent on the number of people who are playing. If twice as many people started joining FIDE rated tournaments, Magnus Carlsen's rating would be much higher than it is today.

Elo ratings don't indicate a fixed level of skill, but are to some extent dependent on the number of people who are playing. If twice as many people started joining FIDE rated tournaments, Magnus Carlsen's rating would be much higher than it is today.
Elo ratings are dependent on the number of people playing? How does that work?

Elo ratings are dependent on the number of people playing? How does that work?
If you can beat a player 100% of the time, your rating will be around 400 points higher than that player. So if you and Magnus are trapped on a desert island, and you start up a new chess federation, he could have rating of 1400 for example, and you or I would be 1000. If two more players arrived on the island, such that each could beat the next weaker one 100% of the time, Magnus's rating rises to 1800 even though his skill level doesn't change.