Quality is far better than quantity.
I ended up with a pretty full A4 size exercise book as my journal, although one of the girls in my school had five. Yes, five!
Provided you stick in all your drafts and are constantly researching/reading, reviewing, and reflecting (the three ee2 r's), you will have at least a decent-sized journal (even if you think your concept doesn't really need to be researched). For those who need to research a lot more (say, if you were researching a culture, for example) then your journal will be a lot more.
But the fact remains, dear hearts. The journal serves to prove that you wrote your major work, and that you did your research to support your major work's development. So long as you do this, it doesn't matter how many journals you have.