What happens if I play Hapu with less than 6 cards in my deck?
You would put 2 cards (or 1 if you only have 1 card in your deck) in your hand and discard the rest.
Source: Unified Minds FAQ (2019-07-19), TPCi Rules Team (2019-08-01)

Relevant cards

Note: All applicable cards may not be listed below.
SM11_200
SM11_200