When I use Zangoose's "Head Hunt" attack, do I have to put all Pokemon I find into my hand, or can I choose to discard some or all of them?
Since you get to choose which Pokemon to reveal, you can choose to discard some or all of them.
Source: Team Up FAQ (2019-01-18), TPCi Rules Team (2019-01-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM9_132
SM9_132