League broke the 5 million concurrent players mark about two months ago.
Source
Multiple leagues exist for the same tier. It's the exact same model as Starcraft 2, except Starcraft 2 caps you at 100 players per tier, and League has 5 separate divisions within each tier. You can still have multiple tiers (e.g. multiple Diamond leagues for both games).
The exception exists only at the Challenger (LoL) and Grandmaster (SC2) level, where only one division exists for both, and is capped at 50 and 200 respectively.
After playing so much SC2, League is no longer stressful, even at the diamond 5/platinum 1 level, lol...