Physically, you have 1 site.
You have 500 IP cameras, so let's say you have 500 accounts.
If you need to reconfigure the site for every camera, you can easily do so.
If you need to change the DNS for your FTP site, you can easily do so.
With 500 sites, you will need either 500 unique binding IP addresses, or 500 unique domain names... otherwise your server will not know where to direct the traffic.
If you need to change the FTP settings for every camera, you will need to change it... 500 times.
If your IP address changes and you need to change your DNS, you will need to do it... 500 times.
For your own sanity, don't make 500 FTP sites.
If anything, you could make 1-10 FTP sites.
Let's say...
ftp.indoor.ipcamera.com - 100 camera accounts
ftp.outdoor.ipcamera.com - 200 camera accounts
ftp.corridoor.ipcamera.com - 50 camera accounts
ftp.vantage.ipcamera.com - 15 camera accounts
ftp.bottleneck.ipcamera.com - 25 camera accounts
and you could group your cameras in different categories to make management easier.
It's all about making it easier for YOU.