In my experience, it's a relatively rare occurrence, but then I haven't been on a committee for quite a number of years.
I used to sit on this committee. I chaired it at one time, but that was a long time ago. I don't remember any leaks occurring from the procedure and House affairs committee, as it then was. It's not a subject on which I'm expert by any means. As for leaks, I've heard of them occasionally, but not very often.
And of course even if the pages are numbered, people can repeat what they remember off the pages. People who have seen material in the committee room can repeat it, and I assume there could be a leak for that reason. It's not something that's easy for committees to control. They rely on people to keep their mouths shut, and usually that works, I think.