After FUE, is it correct the donor area will replenish approximately 25% of the grafts that were harvested from it?

Grafts that are taken from the donor area do not grow back or replenish. Everyone has a finite number of grafts available and they don't ever increase over time. This is why using them efficiently is so important during transplants, and why its usually a good idea to leave some left over in case you ever want a future transplant.

The hair harvested for a transplant will not grow back in the donor area.  It will grow in the new location.