Why is 18765 chosen as the port number? Does it make a difference what numbers I chose?

As far as I know the standard port for the FTP protocol is 21. However, one can choose another arbitrary port number for the FTP service. This depends on the chosen hosting solution or on your server-side configuration (if you deploy/manage your own server-side software stack).

I assume that by using a different arbitrary port number (like 18765 in this case) helps a little with the security concerns (a kind of security through obscurity kind of thing). The idea is that somebody who wants to knock at your FTP door trying to snoop around and maybe attempt a brute-force attack on your password would first try the standard port (i.e port 21) and see that no FTP service responds and would probably assume that there's no FTP on that server and move along. It pretty much amounts to yet another piece of info you need to have an advance (besides your user name and password) to access your files via FTP.

Conclusion: it is a server-side configuration of the FTP service chosen in advance by your hosting solution or by yourself (if you control the server machine).