Thanks ;) I long gave up on making it a trading website, but it serves that purpose as well, and I think it greatly complements other websites by providing a lot of useful information in a nice package. ;)

It's already in there..? In my Tradables page, I can see that number on the right, in a gray box that represents a trading card.
Or can you be more specific?

I was talking about the results from doing a "Bulk Add" operation, either with Tradables or the Wishlist. Here is a sample screenshot:

https://imgur.com/a/ta3gCXW

It would be great if the number of cards could be added to each entry. Maybe in the top right corner?

I'm not sure why there'd be a point in that? You're going to add these entries to your lists, aren't you? Then you just sort the list by Recent, and you can see the number of cards for everything you need. If you're unhappy with something, you can delete it from your list later. (Use the checkboxes, of course.)

I can easily do it but adding more data where not useful is only going to be more confusing. I need the website to remain usable. If you can give me a use case for this...

My use case is that I want to add all games with cards that I don't already own to my wishlist. I coded a quick script in PHP that was able to give me the list of all games with cards straight from Steam and Steam Card Exchange. Now I'm in the process of manually adding them to my wishlist. I agree that the use case is fairly narrow, I just thought somebody else might benefit from this.

The other use case would be when getting a trade offer in the form of a text game list (from, say, Reddit) it would be useful to filter out the games without cards (since I don't want them but it could be because of their lower perceived value).

However I understand that it would be too much resource drain for the limited return. Thanks for replying!

It's not about the resource drain, it's 100% about the on-screen clutter. I'm already quite unhappy with how busy some pages look (such as the library pages), and thinking of ways to make them easier to go through. (I thought going for divs instead of tables would help, but it actually made it worse. Not in a hurry to touch this again...)