Legion Monsters (known as Mass-Production Monsters in the OCG) are a type of Effect Monster. Unlike most regular monsters, Legion Monsters do not follow the restrictions of regular Monster Cards. They do not have any sort of limit on how many you can use in a Deck, in terms of both the number of each individual copy of a single card, and in how they count towards the limit of cards in a Deck. They all possess the following text in parentheses:
(You can have more than 3 copies of this card in your Deck. This card does not count towards the minimum of 40 cards in a Deck. Cannot be used in a Duel by humans.)
The most notable users of such monsters are the Mek Empire in the YRP-Verse.