Well, this is to be expected; the BIOS Data Area has only four slots for I/O addresses of serial ports, with slots for parallel ports immediately following. In the MEMORY.LST file from Ralf Brown’s, we can find the following entries0:
MEM 0040h:0000h - BASE I/O ADDRESS OF FIRST SERIAL I/O PORT
MEM 0040h:0002h - BASE I/O ADDRESS OF SECOND SERIAL I/O PORT
The BIOS list only contains addresses of up to four standard 8250-type COM ports found at boot at the standard addresses.
It will not contain more than four ports, it will not contain any non-8250 type COM ports, and COM ports at non-standard addresses, such as PCIE COM ports, and USB COM ports which don't have an IO address to begin with. It will not ...
The BIOS data area only has room for addresses of four com ports, at 40:00 through 40:07.
More serial ports would be driven by some device driver with some other place to store the address(es) and IRQ(s).