Well. You can have as many cards as you like, but still just 4 copies of any card, apart from basic lands. So there's very little benefit to having 200 cards in your deck.
http://gatherer.wizards.com/Pages/Card/Details.aspx?name=bat...
444 Shadowborn Apostle
1 Reaper from the Abyss
1 Sire of Insanity
2 Griselbrand
4 Cavern of Souls
214 Swamp
There are probably better lists but I like the simplicity of this one and the 666-card deck.
Source: http://www.channelfireball.com/articles/frank-analysis-is-pl...
If you are playing Limited (i.e. Draft or Sealed decks) then you can have as many copies as you have in your card pool.
Although you may have some trouble finding 200 cards to use in a Limited event :)