Cheapest priced copy (Commander Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Knighted Myr 
Artifact Creature — Myr Knight · 2/2 · Common · Commander Masters · #36
{2}{W}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.)
Whenever one or more +1/+1 counters are put on this creature, it gains double strike until end of turn.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 36 | Common | $0.35 | $0.49 |
| Modern Horizons 2 | 17 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 18 June 2021You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.
- 18 June 2021If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.