Perhaps another way of looking at this, would be perhaps a file size limit?, rather than a (amount)file size limit.
The reason I say this is because I currently have about 20 books on the main memory.
However I have around 50 30MB CBR files that have been converted to LRF. I have had numerous problems with tagging and reading files, and with numerous attempts at factory resetting, formatting SD cards & PRO DUO cards, I've just about had enough.
My first initial thought was that Calibre was causing Indexing errors, and indeed it was. Moving too many files onto the PRO DUO card at one time would totally muck my index up.
However after just sticking to the basics (limited tagging, drag and drop files) then allowing the reader to full parse the changes I was able to almost double the amount of readable files on my Card.
Experimenting with higher amounts of files, I discovered a plateau, around 100 files. which equates to about 3GB of additional files. Since then I have reduced to just a core 50 LRF comics, because I wanted to use the Collections feature in Calibre.
I'm pretty positive that there is a bug with the reader as it attempts to re-index a certain amount of data. In my case the rendering of the files was not to blame (since all they were, are <img> tags one after the other).
This might explain the differing variables of books that people are reporting before the reader starts to struggle. And perhaps the issue with cheapo SD/PRO DUO cards.