a clear and technical explanation with regards to the multiple concurrent FTP sessions issue when locking the data port to only one port is what I am most interested in knowing in depth. When can it work, when will it not work, why it may not be recommended, etc.
This will be a wild guess, as I haven’t tested it, you should try it for yourself and see if there are some other problems I might have missed.
I suppose you could limit the passive port range to one single port. In fact you can see in this question that small port ranges are used in practice. Theoretically, to support multiple concurrent connections you only need the 4 values: local IP, local port, remote IP, remote port to be unique. This is how you discern between different connections.
If you lock down the port on your server to one single value, then the only variable left is the port used by the client. This is not a problem, as long as the client has a large enough pool of free ephemeral ports to choose from. Unless it’s doing some heavy NAT, you don’t have to worry about this. Now, be warned this will be purely theoretical stuff: if you used multiple ports on your server, you could multiply the number of hypothetical concurrent connections by enabling number of ports in range connections per one port client-side. But it won’t happen in practice, as I doubt there’s any implementation of an FTP client that would support this (because it doesn’t make much sense). Plus if the client has to share his ephemeral ports in this way and can’t just open a new one, then he has much more severe problems to deal with. So, from this perspective you should be totally safe using a single port.
Let’s think why a single port may not be sufficient.
First of all, I could come up with a situation where a really buggy FTP server implementation uses solely the local port number as a way to identify the client data transfer. Once again, I don’t think any decent FTPd would do that.
The real problem (yes, you can disregard all above as a major digression ;-)) is that passive port range is in a non-privileged range.
This means that your selected port number is not reserved per se, and in fact any user process (doesn’t need root privileges) can grab it before your FTP server does. If you have an abundant pool of ports to select from, you just grab a random free one. If you’re bound to use the only one and it’s already used, you won’t be able to handle the transfers properly.
Sorry, if the answer seems a bit too speculative. To be honest, I tried hard to find a reason why you shouldn’t use a single port and, apart from the last bit, I couldn’t think of any hard evidence against it. Nevertheless, an interesting and challenging question you pose.