+1: Reveal the top two cards of your library. An opponent chooses one of them. Put that card into your hand and exile the other with a silver counter on it.
−1: Put a card you own with a silver counter on it from exile into your hand.
−2: Create a 0/0 colorless Construct artifact creature token with "This creature gets +1/+1 for each artifact you control."
4/27/2018 A token created by Karn’s last ability will count itself, so it’ll be at least 1/1.
4/27/2018 Karn is colorless but not an artifact. The Construct tokens created by Karn’s last ability don’t count Karn.