Most played cards in Mono Black:

2 Decks
Hymn to Tourach
6 copies - 3 average
Sorcerous Spyglass
4 copies - 2 average
Tidehollow Sculler
4 copies - 2 average
Gravecrawler
4 copies - 2 average
Wayward Servant
4 copies - 2 average
Liliana of the Veil
4 copies - 2 average
Wasteland
4 copies - 2 average
Swords to Plowshares
4 copies - 2 average
Carrion Feeder
4 copies - 2 average
Mishra's Factory
4 copies - 2 average
Cryptbreaker
4 copies - 2 average
Marsh Flats
4 copies - 2 average
Surgical Extraction
3 copies - 1.5 average
Mox Diamond
3 copies - 1.5 average
Undead Augur
3 copies - 1.5 average
Chalice of the Void
3 copies - 1.5 average
Liliana's Triumph
3 copies - 1.5 average
Ashiok, Dream Render
3 copies - 1.5 average
Urborg, Tomb of Yawgmoth
3 copies - 1.5 average
Sinkhole
3 copies - 1.5 average