Cheapest priced copy (Homelands): $0.35 at Card Kingdom, near mint, their price list as of .
Clockwork Swarm 
Artifact Creature — Insect · 0/3 · Common · Homelands · #104
This creature enters with four +1/+0 counters on it.
This creature can't be blocked by Walls.
At end of combat, if this creature attacked or blocked this combat, remove a +1/+0 counter from it.
{X}, {T}: Put up to X +1/+0 counters on this creature. This ability can't cause the total number of +1/+0 counters on this creature to be greater than four. Activate only during your upkeep.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Homelands | 104 | Common | $0.35 | — |
3 rulings from Wizards of the Coast
- 4 October 2004If the ability to add counters resolves when there are already the maximum number of counters on it, any counters over the maximum are simply not added.
- 4 October 2004Can attack or block even if it has no counters.
- 4 October 2004Loses a counter even if it is affected by a Fog-like effect which prevents it from dealing damage.