Mikey wants to investigate if there is a relationship between the number of tokens he plays and the number of tickets that he earns toward prizes at the arcade.

On $10$ different occasions, he recorded how many tokens he had played and how many tickets he had earned. After plotting the ordered pairs $(x, y)$, where $x$ was the number of tokens and $y$ was the number of tickets earned, he saw that the relationship was roughly linear.

In conducting a $t$-test for the slope of a the least-squares regression line, how many degrees of freedom should Mikey use?