The Mana Count is one of two indicator exclusive to Mana Monsters, the other being the Mana Energy. It is represented by a green crystal resembling one of the Pendulum Scales, and is located on the left side of the box containing the Mana Effect.
The Mana Count is used to determine how many turns a Mana Monster must be face-up in the Extra Deck before it can be Mana Summoned. For example: if a Mana Monster's Mana Count is 4, it must be face-up in the Extra Deck for two of your turns (including the turn it was placed on the Extra Deck) and two of your opponent's turns. It should be important to note that a Mana Count is always an even number, from 2 to 12. As such, cards and effects that would alter the Mana Count can only change it to an even number.