Is the prostration of forgetfulness one prostration or two, and what is the ruling on the prayer of someone who performed only one prostration?

The prostration of forgetfulness (Sujud al-Sahw) consists of two prostrations. If one inadvertently omits one of them, he must perform it. A single prostration is not sufficient, nor does the prayer become void by omitting one prostration unless one intentionally limits it to that.