My softcover edition of "Surely You're Joking, Mr. Feynman!" has the following at the page 122:

So I went through the entire plant. I have a very bad memory, but when I work intensively I have a good short-term memory, and so I could remember all kinds of crazy things like the building 90-207, vat number so-and-so, and so forth.

It looks that I was wrong when I wrote that she can retain seven things. The seven things limit deals with items in the immediate attention, but evidently the short-term memory goes further than that. By definition it's maintained by agitated neurons, and a brain has billions of those.

Chihiro actually sneaks in a small expose on the topic somewhere around ep.4, when she explains her techniques of retention to Renji. I thought it was a little unrealistic, but perhaps not so much.