Cheapest priced copy (Conspiracy: Take the Crown): $0.35 at Card Kingdom, near mint, their price list as of .
Akroan Hoplite 
Creature — Human Soldier · 1/2 · Uncommon · Theros · #185
Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of attacking creatures you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conspiracy: Take the Crown | 197 | Uncommon | $0.35 | $0.49 |
| The List | CN2-197 | Uncommon | $0.35 | — |
| Theros | 185 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 15 September 2013Once the ability resolves, the bonus won't change even if the number of attacking creatures you control does.
- 15 September 2013Count the number of attacking creatures you control when the ability resolves to determine the value of X.