If this game uses the same random formula as dota 2, then every run increases the drop chance by 1.5%. True hard pity is 100/1.5 = 67 runs.
But no one is that unlucky.
5th run: 7.5%
6th: 9%
10th: 15%
15th: 22.5%
20th: 30%
You don't just fail 15%-30% drop chance 10 times in a row.