I was super curious about the same question.
I think the theoretical maximum is 13 hearts remaining. This is pretty unrealistic though as you have to find the 3 gold treasure chests + the egg + the elf before using your first Heart Scroll. The elf would be nearly impossible to find by this point.
A more realistic maximum is probably: a) find the 3 chests + egg before using your first Heart Scroll, and b) finding the elf at the same time as you kill the Mine King & all mines. In that case the maximum would be 11 hearts remaining.
9 is a really good score!