These numbers are estimates and generalizations.

Land: 39

Ramp: 5

Ramp Doubler: 2

Mana Dump: 1

Removal: 6

Wipes: 2

Draw: 7 (2 pay 2 draw 2)

Tutors: 4

Colony: 30

Good Rats: 2

Rat Synergy: 7

General Synergy: 2

Anthem: 2

Other: These are things you should look at when deciding on things to cut.

Eldrazi Monument

Graf Rats

Midnight Scavengers

Typoid Rats

Rancid Rats

Death Denied

Hall of Triumph

Some thoughts:

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Having more ways to give all your rats evasion could be helpful becuase there are some decks that are pretty good at making chump blockers. Cover of Darkness and Mogis Marauder fit the bill for this. Mogis is especially good if you have a way put tons of rats on the field at once so that they benefit from the haste. Things that give menace would also be good evasion and might be easier to find.

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You need some more ramp and around half your ramp conflict with eachother in regards to the rats. Put some more traditional mana rocks in there. Unstable Obelisk is good to have so you can get rid of those pesky enchantments and artifacts.

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I see tragic slip in the sideboard. Try and find room for it. The -13/-13 is easy to get and helps get around indestructible.

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Put some ways to put tons rats on the battlefield from your graveyard. Those cards will help you come back after you've been shut down. Probably want at least 2 of these: Living End, Patriarch's Bidding, Thrilling Encore, Living Death.

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Pack Rat is one of the best rats ever printed. Put that bad boy in.

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Gray Merchant of Asphodel is rarely bad to put in a permanent based black deck.

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Ink-Eyes and Throat Slitter are good rats to try and fit in here. Burglar Rat is also a neat effect.

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You have 2 cards that are 2 mana and draw you 2 cards. Might be worth to upgrade them to something that draws 3 cards for 4 mana or lets you scry 2 before you draw. Read the bones and Ancient Craving are examples.

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Might be good to have dark prophecy to have a way to keep your hand and board full.

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The less anthems you have the better skull clamp will be, and pure anthems are not really better than just having another rat colony.

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Reliquary Tower is never bad.

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If you are looking for another board wipe. Mutliate is a good cheap one.

General notes around this kind of deck:

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You are probably going to want around 30 Rat Colonies if you want to draw rats reliably. That combined with around 38 lands means you have 32 slots to fill with cards. Some rat colonies can be cut for better rats. This deck will need A LOT of card draw types effects to make sure it can keep playing rats. Make sure to have atleast 8-10 pieces of ramp and around the same for removal. While you don't NEED lots of anthems around 6 is probably plenty. Atleast 2 board wipes will be good to have unless you think you can outrace everyone at once. See if you can piece together any synergies to make it possible to do plays that are better than just playing 3 rats in a turn.
Cards that should definitely be cut:

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Altar's ReapDeathmask NezumiDirty WereratDiseased VerminFoul-Tongue ShriekGruesome FateGutter SkulkInfected VerminKuro's TakenNezumi CutthroatNezumi RoninRats of RathRazortooth RatsRelentless RatsSewer RatsSinous VerminTendrils of CorruptionBlackblade ReforgedBontu the GlorifiedAscendant Evincar

Notes about bad cards and notes about interesting cards. The generically good ones I left alone

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Blackblade Reforged doesn't fit too well in this deck. Since it is only decent for the few legends you have.
Cryptic Gateway is probably only good if you have an easy repeatable way to draw many cards.
The cost of sacing a creature with Altar's Reap doesn't put you up on any cards and waiting to use this to sac a creature that is going to die anyways won't come up reliably enough in EDH. It is probably better to pay 3 mana for something that also lets your scry or 4 mana for something that lets you draw 3 cards.
Ascendant Evincar is a good enough effect if you want something that can hate on small token strategies from your opponents but if you only want it for the +1+1 then there are cheaper things you should be casting instead.
Blood Artist and Zulaport are good cards if you have easy ways to get your creatures back again and again or draw many cards. You also have to have multiple ways to reliably sacrifice your creatures in order to make full use of Blood Artist or zulaport. They are minorly useful otherwise.
Bontu the Glorified would mostly be a beater in this deck. Blood Artist and zulaport can basically do what he does for free.
Bontu's Monument is decent ramp with a pretty decent upside in this deck.
Coat of Arms is really good at what it does. Just be aware that it affects your opponents creatures too. So you probably don't want to play this if there is another tribal deck that is currently ahead on board or can easily make more creatures.
Crypt Rats is a rat that has a pretty useful ability for a creature. Although as it stands most of the time you use this it will wipe your own board so unless you get ways to protect your creatures you can treat this as a board wipe.
Dark Ritual can be good if your deck can be explosive. In this deck it will be decent early game and mostly bad late game.
Death Denied is a good way to refill your hand with what you want. Being able to play this at instant speed is what makes it good.
Diabolic Edict is good at what it does, but in multiplayer formats like edh it is usually better to replace this with a creature or spell that makes everyone sacrifice.
Diseased Vermin is fun but I wouldn't expect it to ever do much damage if it doesn't come down turn 3 or earlier.
Foul-Tongue Shriek at a best case scenario deals like 7 damage, at any point this is good it would have been better to just have another Rat Colony on board.
Graf Rats and it's meld card are bad but it will be so fun when this comes together that it will be worth it.
Gravepurge is probably bad or mediocre most of the time, but if you put Thrumming Stone in this deck you can really go off.
Infected Vermin is a bad version of Crypt Rats in every sense.
Lab Rats is a good late game mana sink. Always pay the buyback.
Rancid Rats and Typhoid Rats is on the cusp of being ok. Deathtouch is a great way to prevent people from attacking if they care about their creatures. Testing will determine if it would be better to just have a high powered Rat Colony isntead.
Rats of Rath seems hard to make useful. There are much better things if you just want a way to sacrifice creatures.
Tendrils of Corruption is using black's hardest way to kill creatures. There are other better kill spells that gain life.
Relentless Rats would only be good if it was the entire focus of the deck.