Realistically can a few glasses of grapefruit juice per week cause negative health effects with 20mg Lipitor (atorvastatin) daily? Or do you need to drink much more?

This is variable. Remember ...Grapefruit juice is very concentrated & this can interfere with metabolism of Lipitor (atorvastatin) ...Best to avoid!
Grapefruit juice. It is becoming clearer and clearer with every year that if one is on powerful medication that grapefruit juice and grapefruits cannot be consumed. Seriously.