I found this forum post about chances of getting legendaries.
Quote:
1 Rare each 0,85 to 0,90 packs
1 Epic each +- 5 packs
1 Legendary each +- 20 packs
Golden:
2% Chance of Golden Common (1 Every 50 Cards)
1% Chance of Golden Rare (1 Every 100 Cards)
0.25% Chance Golden Epic (1 Every 400 Cards)
0.05% Golden Legendary (1 Every 2000 Cards)
Where a golden legendary is 0.0005% chance per card in a pack.
I assume getting 5 golden legendaries would be 0.0005% x 0.0005% x 0.0005% x 0.0005% x 0.0005%
.
But I'm not sure.