How does awarding of bonus starts work exactly?

When the new ranking system came to be, I was awarded the 6 stars bonus due to my rank. I played through that month and the bonus disappeared when I reached gold 5. I finished that month on that floor (gold 7 or 8) and for this past month again I got 6 stars bonus. Now here I thought that these bonuses were tied to the max floor you reached, so I made an effort and made it to platinum 9 this past season, but today I got again 6 bonus stars. What is the formula for these bonus stars?

When the new ranking system came out it awarded bonus stars based on your MMR or highest rank achievement, whichever higher.

So if you've finished gold 7 but your MMR is apparently higher, then you are given 6 stars. Now you've finished platinum 9, you are given 6 stars, the max number of stars you would get anyway. If you do better this month, then you can look forward to 7 stars etc.

The MMR system is not revealed, so there's no way to learning how to lower it (raising it is obvious, just going up ranks). What we know is if you miss out a few months, you still get some bonus stars. How much is not known.

FYI I can tell you I've got consistently 10 stars for Diamond5.

And then this month 10 stars again despite Legend.

The reason is I start playing offmeta when I reach my goal, so I lower my MMR significantly enough.

So you need both ranks and good MMR in order to maximize stars.

Many wild players who finished rank 500-600 in legend were awarded with 11 stars. Me included. So it must be not only about ranks but also about the mmr. 

At each interval you gain another star to a max of 10 at Diamond 5, which I think was equivalent of the old Rank 5.  But I could be wrong.