Well...the maximum array size for variables isn't a problem, you can do it like Npc 1, Npc 2, Npc n°, as it was demonstrated with the custom hats npc...
Yet i never had any problem collecting cards of any tipe (even the mvp ones). With a base lvl of 256 you are nearly invincible against every mob in the game, so it's not hard to just go and get any card you want, making that npc pretty useless, plus there are way too many cards to script that monster of npc(s).
However, I think it would be good for some low-spawn non-miniboss or non-important miniboss cards like tyrfling, zealotus and so, but I think it would be more than 3 ocas....much more xD