From the back cover:

Contains 193, durable, laminated cards for a range of deadly monsters of Challenge Rating 0 to 5. With game statistics on one side and evocative art on the other they are the perfect tool to help Dungeon Masters manage and reference their menagerie during play.

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Correction - the actual count of cards is 177 from the first printing - both in count as well as back of box - other images reflecting count of 193 was either the promotional or otherwise corrected count before printing (71 large carded creatures and 106 small carded creatures).