After making the roll, the number of successes is equal to the number of dice that have a value that is greater or equal to the difficulty. For example: Imagine that you are rolling four dice, and the difficulty is six. You get: 5, 3, 6, and 9 from the roll. As there are two dice that got six or more (6 and 9), you got two successes from the roll.
Please note this result can be affected by botches and excellent successes.