I think at least two would be reasonable. With them, the number of mana can be reduced for more Zombies.
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You should get rid of the Chromatic Lanterns and put Ajani's Mantras instead. And put in more mana.
I would use Fog Bank instead of Songstitcher and Serra Avengers due to their quick, low cost.
If you're going to use the Spawnsire, I surgery you set up a sideboard or something similar.
If you are going to play such high cost cards, you should throw A LOT more mana into your deck.