What is the mode of this data set? 9,14,18,9,12,14,6,6,2,8

The mode is the value that appears more often in a dataset. In your dataset, you have two 6's, two 9's and two 14's.

So, there is no value that is repeated more than any other.

In those cases, some say that your dataset has no mode. Others say that your dataset has three modes: 6, 9 and 14.

It's really all about definition, so it depends on which convention your book/course/professor sticks to.