As many as you want (which boils down to "8" for having a 100% chance of getting exactly 4 of that card on your hand), unless in ELO mode, then 1. N.B.: Copies of one card only will NOT trigger the clan bonus. (You need at least 2 DIFFERENT cards if that clan.)
[A deck that has an optimised chance of getting one of two cards you absolutely want would be: 3/3/2, i.e. 3 times one card, 3 times another card, 2 times a 3rd card. In this way, the bonus is always triggered.)
What I now wonder... What would be about 8 Morpuns? Strictly speaking, the clan bonus (Cancel Leader) should not trigger but I have the feeling that this would be really exploited if it was possible. So maybe the above does not count for leaders and they cancel copies of each other as well.
Which still does not prevent you to have 8 of them in your deck.
Not true, 0Propher-0. Simply not true. Care to give a reasoning for such a statement? I mean, I can say as well "Eating choclate does neve do anything good" or things like that... that does not make them true. Like that, they are only opinions that might not even be deducable.
Reduplicate over-powerful cards and you have really high winning chances. I mean, even 8 skrummxT (or whatever he is spelled) is hard to overcome... similarly with other cards, and that excludes gaining the bonus as would be easily possible with 3/3/2, i.e. only 3 different cards.
Hermeneuticaly: if it would not be any good - why is there a ELO rule forbidding that?