User blog comment:Alpha654/Legendary Cards: What are the Odds?/@comment-25856498-20160922114148

Since it's not listed here, I might as well give a good guess for the Legendary Factor of a Custom Tournament chest. The new ones, ofc. The old ones seemed to be 600, hence why every 1,500 or so cards guaranteed a Legendary; this is no longer true as 2,000 card chests could have a chance of not containing a Legendary.

The lower bound for the Legendary Factor would be 824 (the lowest LF where the chance is less than 100%), and the upper bound seems to be 10000 (Giant Chests have the same Legendary Factor but don't seem to guarantee Epics). Nonetheless I would go and guess it would be 2,000. Not a confirmed or official stat, but it seems like a decent guess to me.