You cannot store too many files in a single folder. As you add more and more files to a single folder, it will become slower and slower accessing the folder. Most users will not notice the limitation. However, some users use automatic surveillance camera or key Logger software that keeps uploading more files into the same folder. Using such automatic software or device is not supported for free service users. Using KeyLogger software that illegally monitors other people's computer is prohibited.
If you have a regular folder that has a huge number of files (subfolders), you can simply create another folder or create a sub-folder, then move half of the files (subfolders) to the new folder.