You may cast any number of legendary instants in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name)
That said, with the 20-card deck limit and so many great supporters, it's getting harder and harder to find room for cards like this
Fiend Effect C Common R Rare SR Super Rare Privacy Settings To use Yu-Gi-Oh
Morska, Undersea Sleuth Commander of the Deep Clue Sea precon, Morska, Undersea Sleuth both rewards you for extra draws and also facilitates doing so with free Clue tokens every turn
His ability allows you to put the top card of your deck into your life when you hit 0 life cards, for the cost of discarding a card from your hand
It gives all your creatures +1/+1, trample, and infect until end of turn