Silly Question

[HerenIstarion]

No textual evidence as to page number, but number of chapters is known (more or less):

LoTR Book VI, Chapter 9 Grey Havens

In the next day or two Frodo went through his papers and his writings with Sam, and he handed over his keys. There was a big book with plain red leather covers; its tall pages were now almost filled. At the beginning there were many leaves covered with Bilbo’s thin wandering hand; but most of it was written in Frodo’s firm flowing script. It was divided into chapters but Chapter 80 was unfinished, and after that were some blank leaves

Blank leaves were for Sam, and it is told earlier that the whole chapter was also dedicated to Battle of Bywater.

If we count what is now appendices, genealogies and other stuff as included into original 80 by Frodo, than it gives overall number of 81 (speculation)

It is interesting that LoTR as it is published has 62 Chapters proper, + 4 of the Prologue + roughly 17 subsections of appendices, it sums up to 84. If, on the other hand, we count Prologue as one chapter and not four, it gives same number of 81.

Assuming the following conclusion on the previous coincidence, I may utter a guess tha Red Book was approximately of the same length as LoTR in pages, a little bit more maybe because of Bywater chapter.

As for method of compilationi, probably he added pages as he needed them:

LoTR, Book VI, Chapter 6 Many Partings

[Bilbo:] I wonder, Frodo my dear fellow, if you would very much mind tidying things up a bit before you go? Collect all my notes and papers, and my diary too, and take them with you, if you will. You see, I haven’t much time for the selection and the arrangement and all that. Get Sam to help, and when you’ve knocked things into shape, come back, and I’ll run over it. I won’t be too critical.’
‘Of course I’ll do it!’ said Frodo.

Allegedly, the LoTR was Bilbo's diary + all the notes Frodo collected from him in Rivendell, and what he himself and Sam added to it bound together. It seems possible and probable that book was rebound as need came and new pages were added to it.