There are at least 130 rules of use of the Death Note. Do they all fit on the one last page of the notebook (how?) or do they come from some other source? Was it ever addressed in the manga or the anime?
The Death Notes originally do not have any rules written on them. Ryuk dropped a Death Note in the human world because he was "bored", so he only wrote enough rules to get a human interested in it (and he wrote them in English, the most common language). During the course of the plot, Light infers several rules not written in the book through his experiments. (Even Ryuk admitted once he did not know of one such rule.)
Ryuk didn't write all rules in the deathnote, only the most important ones:
"The human whose name is written in this note shall die."
"This note will not take effect unless the writer has the subject's face in their mind when writing his/her name. Therefore, people sharing the same name will not be affected."
"If the cause of death is written within 40 seconds of writing the subject's name, it will happen."
"If the cause of death is not specified, the subject will simply die of a heart attack."
"After writing the cause of death, the details of the death should be written in the next 6 minutes and 40 seconds (400 seconds)."
The other rules exist, but Ryuk never wrote them into Sidoh's book and only explains them.