Artillery refers to large-caliber guns — guns with big barrels — which can be moved from one place to another for land battles. The artillery is also the name for the army unit that uses these big guns.
If you're a military commander and you say, "Bring in the heavy artillery," you're ordering the artillery to move into place and get ready for action. But if you're talking to fellow members of your debate team and you say, “Bring in the heavy artillery,” you mean "bring in our best debaters, our heaviest hitters." Artillery can refer to one weapon or 100 — the singular and plural are the same.