The sufficiency comes from the number of dice rolls.
If you roll one die, you get 1, 2, 3, 4, 5, or 6. (6 options)
If you roll two dice, you get 11, 12, 13, 14, 15, 16, 21, 22, 23, 24, 25, 26, 31, 32, 33, 34, 35, 36, 41, 42, 43, 44, 45, 46, 51, 52, 53, 54, 55, 56, 61, 62, 63, 64, 65, or 66. (6 x 6 = 36 options)
If you roll three dice, you get 111, 112, 113, 114, …, 663, 664, 665 or 666. (6 x 6 x 6 = 216 options)
And so on. Each dice you roll multiplies the total number of options by 6.
So if you roll 100 dice, the number of options is 6^100 = 6.533×10⁷⁷ This is an unimaginably vast number.
Vetting that the rolls produce the seed is secondary. The important point is that each roll multiplies the search space, and 100 is plenty.
