Answer: 9. Solution. Note that the sum of the numbers on each leaf is the same. If the last page has the number $n$, then on the first leaf, the numbers will be $1,2, n-1$ and $n$. Therefore, $2n + 2 = 74$, from which we get that the number of pages $n=36$. Four pages are placed on each leaf, so there were 9 leaves.
2. A stack of A4 sheets was folded in half and folded in two (resulting in an A5 booklet). After that, the pages of the resulting booklet were renumbered: $1,2,3, \ldots$ It turned out that the sum of the numbers on one of the sheets was 74. How many sheets were in the stack?