Q:Can sn EDH deck have any number of Rentless Rats?
The Commander rules state:
903.5b With the exception of basic lands, no two cards in the deck may have the same english name.
I’ve also answered this question before. BUT
112.6m An ability that modifies the rules for deck construction functions before the game begins. Such an ability modifies not just the Comprehensive Rules, but also the Magic: The Gathering Tournament Rules and any other documents that set the deck construction rules for a specific Constructed format. However, such an ability can’t affect the format legality of a card, including whether it’s banned or restricted. The current Magic: The Gathering Tournament Rules can be found at www.wizards.com/wpn/Events/Rules.aspx.
However, your deck still must be 100 cards exactly:
903.5a Each deck must contain exactly 100 cards, including its commander.
This means that if Relentless Rats is ever Restricted outright (specifically) in the format, you can’t run more than one. If it’s banned, well, you know you can’t use it at all.
Short answer: Yes, you can run more than one. But consult your playgroup first though. The community is still kinda debating it. You know, “spirit of the rules versus letter of the rules versus fun” kind of thing.