What is the mode of 10, 12, 11, 10, 15, 20, 19, 21, 11, 9, 10?

I have a set of numbers: 10, 12, 11, 10, 15, 20, 19, 21, 11, 9, 10. I want to know the mode of these numbers, which is the number that appears most frequently in the set.