How much trading cards for a key ?

~20

depends on trading cards, just check the price on the market (you have to take account for valve fees of course).

Cards from games which have been bundled countless times aprox 30-35 cards, Fairly new games and games which haven't been bundled and aren't popular can fetch you a key for 10-15 cards.

depends a lot on cards. atm key is 1.79€. Most common cards sell for 0.05€ (meaning u can get 0.03€ per card) |
1.79 / 0.03 = 60 cards. |
Some rarer cards (less common games) get u 15 cents. |
1.79 / 0.15 = 12 cards. |
So it would be from 12 rare cards to 60 very common cards (like tf2 ones) |

BUT if u sell them for metal.. 9 scrap per refined, 7.5ref = key then it would be.. |
1 card -> 1scrap (almost insta selling) : 68 cards |
1 card -> 1scrap + 1weapon : 45 cards |
1 card -> 2scrap (it could work out with some time) : 34 cards |
If u sell rarer cards u can get much more per card, but those will be harder to sell for real price.

Edit: This calculations are ment to sell cards in steam market and buy the key from there with gains.

Cheers

Trading cards for keys will usually be a bad deal unless you find one of the rare people buying to collect rather than to profit, and selling low value cards on the market eats up most of the profit due to the minimum fees. I recommend using bots such as backpack.tf's and Steam Card Exchange to trade multiple lower valued cards for higher valued ones, then to sell those on the market.

30-35 is the most used value