Really! I have some with eight. Perhaps you are thinking of connecting a mouse to the computer via a serial port. Yeah about the nineties they switched the mouse to PS2 which is a serial port by another name and form. Jetway makes a board with 10 serial ports aboard in micro ATX form. I used one to set up a process control system in a small factory in southwest Washington. Rather slick.
Well they can still be found, although I don't know of anyone making them. It wouldn't be too difficult to do. Let see, I think you could make them to do 256kbps connecting through the ports.
Right you are Sly. I installed a Navigation system for a guy in Portland. He insisted he wanted to use his laptop. I don't like the damn things but Hey! Laptop it is man! The only way to connect his Furuno GP32 GPS was using a a Serial Port Adapter plugged into one of his two USB ports and another plug into the other USB port to run the Auto Pilot. He started to ask how he was going to plug... But he thought better of it, I sort of tried to talk him out of the laptop for a dedicated onboard system. I sort of expect to hear from him again, maybe this summer. Many of the serial ports before win 7 came out and damn near all the USB serial port adapter before that time will not work with win 7 or 8. Damn near rude is what I think of it.