Yes, Blackbeard did eat two Devil fruits. He ate the Darkness matunda (Yami Yami no Mi), and then aliiba Whitebeard's Quake matunda (Gura Gura no Mi). Also, considering the fact that he and his crew are hunting Devil fruits, I think that it is salama to assume that he may be able to eat more.
Most people cannot do this as the two 'devils' inside of them would kill the person, however Oda Sensei has alisema that Blackbeard has a 'special body' so he can do this. Many have theorized that this is because of the Yami Yami no Mi. Perhaps one ability of this matunda is the ability to absorb everything, even other Devil Fruits.
If this was the case, it would explain why Blackbeard wanted this matunda so badly -- badly enough to steal it from Thatch, kill him and be hunted down kwa his fleet commander.
