Cheapest priced copy (Alara Reborn): $0.49 at Card Kingdom, near mint, their price list as of .
Knotvine Paladin 
Creature — Human Knight · 2/2 · Rare · Alara Reborn · #71
Whenever this creature attacks, it gets +1/+1 until end of turn for each untapped creature you control.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alara Reborn | 71 | Rare | $0.49 | $1.49 |
| Duel Decks: Knights vs. Dragons | 10 | Rare | $0.49 | — |
One ruling from Wizards of the Coast
- 1 May 2009The number of untapped creatures you control is counted as the ability resolves, and the bonus is locked in at that time. By this point, other attacking creatures will already be tapped (unless they have vigilance).