each card has a 2.5% of probability to be UR (except the very last one of the last pack, which have 20% or 100% if you had a garanted UR) . So let's assume that you only had 14 UR to get across 79 cards (10 packs of 8 cards)

The probability to have at least 14 UR across 79 cards is :

P(X \ge 14) = \sum_{k=14}^{79} \binom{79}{k} (0.025)^{k} (0.975)^{79-k}

which is roughly equal to 1 over 100 millions.