What is the maximum number of files (or folders) that can be stored in a single folder?

See This link at Microsoft.

It suggests that there is no limit to the number of files in a given folder as long as the number of files on any given volume is not greater than 4,294,967,295 (on NTFS) the link gives much lower limits for FAT32.

Although this isn't an answer to your question, keep in mind that with many file systems performance will start to degrade if a directory has more than X files. In ext3 I think it around 30,000.

NTFS: 4,294,967,295 (Wikipedia Entry)

I don't think there is a limit "per folder.". It should be the same as the absolute limit of files per NTFS volume: 2^32 - 1. It would require 512 byte sectors and a maximum file size limit of one file per sector.

Realistically you have to calculate a realistic average file size and then apply these principles to that file size. So, I wouldn't be preoccupied, I have seen folder with much more that 5000 files. But if you want to open such a folder in Windows Explorer, you could have to wait for minutes. Consider using command line tools for accessing that folder.

Here is an interesting link on Technet: How NTFS Works