Well, that depends on your definition of venomous. Venomous and Poisonous are not the same thing.
Poison: A substance that causes injury, illness, or death, especially by chemical means. (includes venom)
Venom : A poisonous secretion of an animal, such as a snake, spider, or scorpion, usually transmitted by a bite or sting.
Now, presuming you are refering to poison and not venom, i'm not sure of the exact number but the duckbilled platypus (males only), several species of shrews (short-tailed shrews namely, are neurotoxic. also the European water shrew), and the Solenodon (a small insectivore) are poisonous.
So that's about 4? Whats the exact number?