It is not the number of photos it is the resolution the photos are in.
As a rule photos display at 72 DPI (dots per inch), this is great on a computer screen and for storage. 300 DPI or higher is what is usually sent to a printer.
This does not translate to the actual photo size because a photo can be 72 DPI and only 2" square or it can 8". Usually the higher the resolution the larger the size, usually.
What I am saying is what the photo resolution is is where the problem may be.
Select any photo that does sync and any that does not and compare the resolution and size of them.
The total number is XXXXX the issue, it is the format and or resolution.
Best is jpeg format, it is the standard for all, there are many other formats that work well and some that are proprietary to a device or software.
Does that help explain it?
Compare some and let me know what you find, I think there is a way to fix this without jumping though too many hoops.