Cheapest priced copy (Wilds of Eldraine Promos): $10.99 at Card Kingdom, near mint, their price list as of .
Moonshaker Cavalry 
Creature — Spirit Knight · 6/6 · Mythic · Wilds of Eldraine · #325
Flying
When this creature enters, creatures you control gain flying and get +X/+X until end of turn, where X is the number of creatures you control.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
6 printings · $10.99 to $179.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Wilds of Eldraine Promos | 21p | Mythic | $10.99 | $17.99 |
| Wilds of Eldraine Promos | 21s | Mythic | — | $37.99Out of stock |
| Store Championships | 17 | Mythic | — | $179.99 |
| Secrets of Strixhaven Commander | 110 | Mythic | $10.99 | — |
| Wilds of Eldraine | 21 | Mythic | $11.99 | $15.99 |
| Wilds of Eldraine | 325 | Mythic | $19.99 | $29.99 |
2 rulings from Wizards of the Coast
- 1 September 2023The value of X is determined only as Moonshaker Cavalry's triggered ability resolves, potentially including Moonshaker Cavalry itself. It won't change later in the turn if the number of creatures you control changes.
- 1 September 2023Moonshaker Cavalry's triggered ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't be affected.