Grandmaster just means you have reached a certain elo and have completed certain requirements (win a number of games, against certain opponents etc), and won a certain number of competitions. There are around 2000 current grandmasters.

World champion is much much more selective, meaning literally winning the world championship.

Don’t get me wrong, grandmaster is incredibly prestigious, but is much more “common” than world champion.

(International Master is the rank below grandmaster).