The spell False Life gives you a small amount of temporary hit points. If you cast it multiple times, does the temporary HP stack?
Temporary hit points never stack. If you get another source of THP, you either keep your current THP or take the new THP, usually taking whichever is higher. (PHB 198)
It will not stack with temporary hit-points. However, temporary hit-points that are not temporary hit-points (Arcane Ward, Wizard Arcane Tradition: Abjurer) will stack, as it is a seperate entity that behaves like temporary hit-points. It is not THP, but takes damage for you and cannot die. Therefore, it, and as far as I know only it, will stack with False Life. Polymorphing is not THP, but it behaves similarly so in theory the maximum 'temporary' hit-points you could have would be being Arcane Warded, Polymorphed and False Lifed.