Most played cards in Mono Red:

3 Decks
Firebolt
12 copies - 4 average
Light Up the Stage
12 copies - 4 average
Relic of Progenitus
12 copies - 4 average
Soul-Scar Mage
12 copies - 4 average
Lightning Bolt
12 copies - 4 average
Blood Moon
12 copies - 4 average
Monastery Swiftspear
12 copies - 4 average
Seasoned Pyromancer
12 copies - 4 average
Bonecrusher Giant
12 copies - 4 average
Blast Zone
11 copies - 3.67 average
Flame Slash
10 copies - 3.33 average
Bomat Courier
8 copies - 2.67 average
Lava Dart
8 copies - 2.67 average
Pillage
7 copies - 2.33 average
Shattering Spree
6 copies - 2 average
Obosh, the Preypiercer
3 copies - 1 average
Kozilek's Return
2 copies - 0.67 average
Blistercoil Weird
2 copies - 0.67 average
Pithing Needle
2 copies - 0.67 average
Oblivion Stone
2 copies - 0.67 average