Cheapest priced copy (Conspiracy: Take the Crown): $0.35 at Card Kingdom, near mint, their price list as of .
Strength in Numbers 
Instant · Common · Modern Masters 2017 · #138
Until end of turn, target creature gains trample and gets +X/+X, where X is the number of attacking creatures.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
8 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conspiracy: Take the Crown | 193 | Common | $0.35 | $0.49 |
| Modern Masters 2017 | 138 | Common | $0.35 | $0.49 |
| The List | MM3-138 | Common | $0.35 | — |
| Salvat 2005 | J42 | Common | — | — |
| Salvat 2005 | J52 | Common | — | — |
| Salvat 2005 | J53 | Common | — | — |
| Time Spiral | 224 | Common | $0.35 | $0.49 |
| Time Spiral Remastered | 233 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 19 March 2021The value of X is determined only as Strength in Numbers resolves. It won't change later in the turn if the number of attacking creatures changes.