how many cards are expected for a TF2 key?

It really depends, because the value of cards varies. I've had some where the value gained is 3 - 4 cents a card, while others can be sold for $5 and higher.

Given most of the games you have cards for have been bundled, you aren't going to get much for those.

Well let's do the math.
A common card costs around... 0.14cts
2.49*/0.14=17.5 cards, rounding up, 18.

*Official price, and card price to my experience.

It depends on the cards.
Some cards are worth more than others.
From what I know, current average price for non-foil cards right now are about 0.10.
Since a key is roughly 2.45, it will take 25 cards at least (since steam will take a portion of your earnings from the cards).

I'm not going to accept less than 40 cards regardless of cards' quality. TF2 is easy to sell and trade, and it's commonly accepted as currency. Cards, on the other hand, aren't. They can be traded, but with much more hassle.

wow 40 cards seems like a lot. most my cards seem around 10cent to 15 cent value . but I agree, dealing with cards can get annoying.
25-30 sounds about fair it seems like it would come out to more then the value of a tf2 key.
I just never traded for one so I Did not know if there was a standard.

Just so you know, keys can pretty easily be bought for around $2 in stuff. If you have paypal you can get them for about $1.80 from traders.

You'll likely run into overcharging if just using the cards to trade for a key as cards are much more of a pain to move. I would suggest you sell your cards on the market and just buy a key that way. Its roughly $2.40 fluctuating up and down. You would probably be able to do that in a day or two iwth the cards you currently have.