See https://manual.seafile.com/config/seahub_settings_py.html
Max number of files when user upload file/folder.
Since version 6.0.4
MAX_NUMBER_OF_FILES_FOR_FILEUPLOAD = 500
The web browser interface is not designed to handle extreme numbers of files, it’s for one-off uploads and downloads. If you need to do serious syncing, use the local client Windows/Linux/Mac client.