In our case, we are using several USB to serial converters plugged into a couple of hubs. They are supposed to be the same, so it is not the manufacturers fault that they are indistinguishable by anything other than their physical connection. Unfortunately there is no reasonably priced alternative to USB,
A single multi-port USB to serial converter would address this nicely, but
we have evaluated about a dozen of them, and while all they work perfectly in Windows, they are either unsupported in Linux at all, or horribly broken (like dropping a byte every couple of hours). It is an embarrassing shame. I wish our company had the resources to sponsor somebody to fix that.
The only multi-port converter that worked well (other than not having its "non-free" firmware in Debian), and was reasonably cheap and available in the USA, was an old Keyspan 4-port model, which is no longer manufactured...