"Know" is a stretch. Plants respond to attack by releasing chemicals (e.g. nettles and grasses), curling or retracting their leaves (e.g. acacia), or by changing their morphology (e.g. holly); but they have no nervous system - let alone a brain - so it's not like you're killing an animal.
As many as necessary, really. The only reason they start getting removed is because of the risk of infection and stuff being higher than the risk of complications with the surgery after a certain point.
It's complicated, subjective, and probably varies country-to-country.
It's more that there's a higher risk of complications if they remove the old kidneys. However, they do start taking them out if you've had more than two transplants.
I've said it before and I'll say it again: right when the MAGAts seem to have reached yet another limit of human stupidity, they find another new and unexpected way to be complete and utter fuckwits.
