Even without combos, playing exactly 60 cards is good because it makes your deck more consistent. Adding cards beyond the first 60 reduces your chances of drawing your key cards, and adds the possibility of drawing marginal cards instead.
Anyway, I just realized I forgot a few cards that deserved a recommendation:
Compelling Deterrence,
Liliana's Mastery,
Thought Scour or
Mental Note, and
Gisa and Geralf. Also,
Shambling Goblin has been rendered obsolete by
Festering Mummy.
There are a couple of critical decisions you need to make before going forward.
First, what's your budget?
Second, what basic strategy do you want your deck to employ? The traditional Zombie deck is a moderately fast synergistic aggro deck that excels at bringing
dead creatures back for another go, but by playing cards like
Sire of Stagnation you show interest in a slower, less synergistic deck that wins through enormously impactful single cards.
Regardless, it'll help to sort your deck a bit better. Here's a type/cost sorting:
//Lands
2
Submerged Boneyard12
Swamp11
Island//Creatures
2
Cryptbreaker1
Returned Phalanx1
Miasmic Mummy2
Stitched Mangler1
Unbreathing Horde1
Voracious Null2
Liliana's Elite2
Diregraf Colossus1
Gavony Unhallowed1
Hound of the Farbogs1
Lamplighter of Selhoff2
Advanced Stitchwing1
Noosegraf Mob1
Sire of Stagnation//Other spells
2
Graf Harvest2
Spontaneous Mutation2
Cemetery Recruitment1
Unsubstantiate2
Murder3
Rise from the Grave2
Grave Betrayal2
Dark SalvationAs for adding more cards, you're really spoiled for choice. Obviously I've suggested more cards than you can ever use, so it's up to you which to play. Regardless of which path you take, I suggest dropping expensive low-impact cards like the
Lamplighter of Selhoff.
There aren't many hard-and-fast rules about how many of each card to play. But there's a good column on the official site that can provide some guidance, on this topic and on others. Here's the archive:
http://magic.wizards.com/en/articles/archive/117690 (The articles about card-counts are Four of a Kind, Three's Company, Take Two, and One for All.)
PS: When it comes to lands, you can do better than
Submerged Boneyard. Also, 25 lands will be too much if you end up going the fast aggro route.