Potential second FUE transplant: how many grafts in total can be removed from donor area? (Photo)

I've had 4100 grafts fue performed, I'm considering second transplant to thicken up the top, my donor area was considered plentiful and my hair is generally quite thick. I have also included picture of it prior to my fue. My question is really two; first, how many more grafts could I have before my donor area becomes very noticeably thin, and second, how soon after my initial fue could I have another one done given the trauma that my scalp has gone through. I thank you for your troubles in advance