We see in Death Note that the pages of the Death Note are not self-healing, and pages torn out do not seem to be restored. At least some owners possess a Death Note for multiple years, and pages remain filled out with names written in the past (at least by the current owner). Still, the notebook is always depicted as a slim notebook.
If the owner manages to fill out all the pages, or tears out enough pages to need more, does the book somehow add pages, or is the owner no longer to write more names?