You're limited to one S rank chest per champion per season, and you can only stock up 4 boxes to potentially earn per month (1 per week).
The season is almost over, so you'll be able to re-earn chests for all the champs you like when they reset, but until then you'll either have to play other champs, or outright buy chests if you want them.
Since theres limits on how many chests you can earn (as mentioned, 1 per week, although you can save up to 4 potential chests), no one has earned a chest for every champion simultaneously (theres more champs than available chests in a season), however theres lots of people who have managed to earn a chest on a different champion every week since release, myself included.