I know of no tool that will let you count the number of directory catalog entry "slots" in use or available in a FAT32 folder. A nice gentleman named Udi Cohen has a C++ utility for sorting filenames in fat32, and you may be able to define a class that that will let you enumerate or get a count of these entries
The limitation is set by the file system. FAT32 can have 65,536 entries. Each file and subdirectory takes from two to thirteen entries, depending on the length of its name.
The FAT32 spec is available here:
It gives details on everything, including the directory structure and
the way Long File Names (LFNs) are stored in directories.
Basically, there's one directory entry that always holds the short
(8.3) version of the file name. If the actual name doesn't fit that
mold, then there will be one to twelve more entries, each of which
holds 13 bytes of the full name. So a file named
"veryverylongname.txt" would take three entries. The first would have
the short name, something like "veryve~1.txt", the second would have
"veryverylongn", and the third would have "ame.txt".