It should be some top % of MMRs, but as the Wild player pool is so small, not many will get it.
I hope they just make the MMRs numbers available at some point, like they eventually did in Starcraft 2 and already do in battlegrounds. The current ranked Hearthstone just leaves us with a pointless ranking system (up to legend) and a lot of unanswered questions.
Does it matter tho? Don't you get x11 stars when you hit legends regardless of the points? I was like 2600 legend in standard a week ago and haven't played since then, got on it today to get my rewards and still have x11 stars.
Top 800 in Wild and got my 11x. The month before around top 900-1000 and only got 10x. Maybe it was because I also got matched against 200-600 legend guys, when I got 10x i only got matched against diamond 4-1 ... seems like the matchmaking needs to give you high mmr opponents so that your mmr can get high enough for the 11x.
Oddly enough first season of the new system i went to legend(320 legend highest), got a 10x bonus for last month. Last month I only went to Platinum 10, got an 11x bonus for this month, so I believe they just changed it in some way. Either way I couldn´t tell you what exactly it took for the 11x bonus either.
I don't really know, but something is definitely broken in the new system, since I got 9x star bonus on my main account where I had Diamond 1, and I also got 9x star bonus on my second account where I had Gold 7. I think there is some kind of bug, not that I complain tho ;)