When they say roll a d66 or d666, what they're saying is that you roll a d6 for each of the 100s place, the 10s place, and the 1s place numbers. E.x. for a d666, they will roll 3 6 sided dice (for example, let's say 3, 4, and 6 are rolled) and the first number will be the 100s place (300), the second number will be the 10s place (40), and the final number will be the 1s place (6), totaling up to 346. This is why when rolling a d66, the numbers 17-20 (and subsequent iterations) are missing, because each 100/10/1s place will only go to a maximum of 6. Hope this helps!