I also think it's 999. I know it's at least 740 as I have a toon with that, but I didn't max anyone last year. You might be able to see in one of the loot summary tabs. Hover over the Qwardian crown in the summary and see if it says 1/999 or something else. I know it does this on source marks (1/999999) when you get one in a chooser, but I don't know if it will just show on the summary and I'm not sure crowns show up in an actual chooser ever.
I can confirm It's 999 from the Test Server. Every seasonal currency is now 999 max and episode currency too, It's only the Source Marks, Fate Tokens & Destiny Tokens that ends at 999999...
