When installing a coolant temperature sensor how much coolant will I lose?

Quite a bit if you don't drain it down as the ECT is in the head. However there is a way that can minimalize the loss. Get everything cleared out so you have good access. Now loosen the old sensor so you can easily remove it. Now squeeze the top radiator hose and release it, that will create a slight suction in the cooling system. At the instant you release the hose, pull out the old sensor and stuff the new one into the hole. Tighten it down. If you do it right you will only lose about a cup or so of coolant. If you need a bit more time you can squeeze the hose, release and then pull the sensor and stick your thumb into the hole. Then squeeze and release again to install the new one.

Let us know how you make out. I've changed a bunch of sensors that way without an issue. However if you haven't changed the coolant in a while it might be a good time to drain and refill.