...20 or more cards in hand, you get +2 and +2 .
...25 or more cards in hard, you have 4 additional hand slots, which can only be used to hold Weapon, Tome, Relic, Tool, or Item assets.
...30 or more cards in hand, doom cannot be placed on enemies, locations, or the current Agenda.
...50 or more cards in hand, reduce the fight, evade, and health of all enemies in play to 0. Enemies cannot attack you, engage you, move into your location, or spawn at any location.
...100 or more cards in hand, remove the encounter deck and discard pile from the game. Then replace all tokens in the chaos bag with tokens, then remove the chaos bag from the game.
...200 or more cards in hand, record ¯_(ツ)_/¯ in the campaign log, then exile the Collection.
Book of Books
While you have...
- ...5 or more cards in hand, you get +1 .
- ...10 or more cards in hand, you get +1 .
- ...15 or more cards in hand, you get +1 .
15 Cards in hand is very unlikely unless I've missed something that gets you a huge hand size boost and even then you're unlikely to both fill the hand and want to hold them all.
10 is doable with the new size boosters (Looking at Harvey Walters) but a card in hand is one you aren't using
Quite a few Seekers routinely hold 5 cards for their Educational edification however. It's not fast or free but if you're playing with a big hand anyway you might want this over the Magnifying glass (it's a Tome)
EDIT: I missed Dream Enhancing Serum which could get you some very big hands -you're probably still better off just throwing the cards at checks most of the time.