This is one of those "Gotchas" that most people usually learn about the hard way. While it's rare to have more than 10K files in a directory be aware when using regular FTP, there is 10K file listing limit on most servers. It's a default config in ProFTP. Even downloading the directory itself will only get you 10K files. There is no warning or indication.

Note this is per directory, 20K files spread over numerous sub directories are unaffected. Also note it's not a directory limit.

To avoid this use SFTP. Not sure when this became the norm. I know I have used FTP in past to download a directory with more than 10K files many moons ago. I've been using only SFTP for a least the past 10 years.