This is just weird, as all other resources from trees are limited (and it has been argued, rather harshly so) in numbers.
The limitations on the other tree resources serve mostly as PITAs for new players (and to a lesser degree for established ones).
A limitation on mulberry leaves would be an annoyance for established players, and be an interesting mechanic in introducing a new resource in villages: area.
One may argue that it is "realistic" to have a large (but not unlimited) amount of leaves on a tree, but as we all know:
SCREW REALISM!

IMO, 16-20 leaves per tree would be fine (multiple of 4, plx).