In some cases I get 5 epics and in others I have obtained up to 4 legendary ones.
Typically, you get 1 legendary for each premium box. But look! most of these legendaries are fusion material. That is, specials legendary have a much lower drop rate.
And within the specials, there are other even more special ones that are the difficult card. It took me 6 months to get 1 platinum plate.