Originally Posted by
Tacosnape
Two if you're dealing with a lot of Stax/Pox/Wasteland heavy decks. Otherwise just one will suffice.
I currently run a 3/1 Stifle/Extirpate split. I'm tinkering with a 2/2 split, and there's no way I'd ever go lower than this. Extirpate absolutely sucks in more matches than Stifle does, and in a match where both suck, Stifle's better, because Extirpate won't pitch to a Force. Play two and two if you feel too Stifle-heavy. I still like 3-1 for the time being.
Jace's strength is what he does unchecked: He wins. You'll almost never have to use his kill, because the card advantage he'll generate you if he lives long enough to go Draw/Draw/Both Draw/Draw (Or, alternately, Draw/Draw/Draw-die-PlayJace#2-Draw), will steamroll into a very bad situation for your opponent.
At the absolute worst, he cantrips and prevents 2 or more damage to you. Not a great effect for 1UU, but just think of it as a slightly pricier but on color Renewed Faith. With the potential to win you the game if the opponent can't stop it.
EDIT: Also, Meddling Mage is fantastic against White Stax, just for the record.