Skeletal Swarming is a rare Enchantment card from Adventures in the Forgotten Realms (AFR), collector number 232. Current market price: $0.31 (foil $0.29), TCGplayer market prices via Scryfall.
Each Skeleton you control has trample, attacks each combat if able, and gets +X/+0, where X is the number of other Skeletons you control. At the beginning of your end step, create a tapped 1/1 black Skeleton creature token. If a creature died this turn, create two of those tokens instead.