At the end of each week, the sheets are all ranked according to the number of downloads in the past week. Then they're split into 11 brackets and given a 'rank' from 10/10 to 0/10, with each rank having its own colour. So the top 9.09% get 10/10, the next 9.09% get 9/10 etc etc ... and the bottom 9.09% get 0/10.
So to answer your question, there is no set number of downloads to get into a bracket, its just about the last week's rankings.