Quality not Quantity. If you can write a good response and cover everthing in one booklet then your not going to loose marks becuase someone else worte an equally as good essay in 2 booklets, in fact if you can get it out in a smaller amount of space without throwing crap in there then that is great.
It depends on the essay, but I generally have 4-5 paragraphed pages of A4 writing which will work out to be roughly one BOS book and possibly a second. But I'm not fussed about how many BOS booklets it is. Like hollybox said, quality not quantity.
Aim for about 4 a4 pages per essay really.... the quantity of it is so dodgy though, as something that appears to be bigger becuase of big handwriting may actually be smaller than the one with smaller handwriting.