How many cards in a deck is "too many"? Image related

Depends on the game obviously.

When it comes to size, I personally like the size of 30-card decks - when placed on the table, they stand at a good height sleeved or unsleeved.

Anything under 20 looks wimpy when it's unsleeved. Anything higher than 50 starts to become too tall when sleeved. And anything over 80 becomes too difficult to shuffle when sleeved.

>>54312899
Depends on how often you have to shuffle it. If often , then more than 100 is too much.
Else you can go all the way up to 300 before it becomes unmanageable.

>>54312899
The way MtG defines it, a deck is too large if you cannot comfortably shuffle the deck without outside assistance.

In terms of an actual number, I'd say that 60 cards is the sweetspot and 100 cards is pushing it.

given that above 100 is hard to shuffle and acceptable only if it has to be shuffled exactly once at the start of the game...

it depends on the game and what the deck is about.

for TCG like magic the gathering 40 or 50 seems to be a sweet spot, but Magic the gathering needs more because one third of the cards in the deck are utterly boring lands.