16.3 Number of Serial Ports Supported
If you have more than 4 (or possibly 2) serial ports, then you must insure that the kernel knows this. It can be done by configuring the kernel when compiling or by a parameter given to the kernel when it starts (boot-prompt or kernel command line).
The kernel configuration parameters: CONFIG_SERIAL_8250_RUNTIME_UARTS=4 and CONFIG_SERIAL_8250_NR_UARTS=4 set the maximum number of ordinary serial ports (UARTs) equal to 4. If you have more than 4 ordinary serial ports, then you need to change the 4 to whatever. But you may override this via the kernel command line for example: nr_uarts=16 (if serial support built into the kernel) or 8250.nr_uarts=16 (if serial support is via a module). The boot loader such as lilo or grub can be told to do this.