Problem is Ubuntu does not recognize more than 4 ports by default.
To solve this you need to add a kernel boot parameter to instruct the kernel during boot to look for more serial devices.
With Grub2 on the machine, you can no longer edit the grub kernel parameters directly as Grub2 uses a templeting system.
So use your favorite editor and open (as root)
there you will see a line
change it to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash 8250.nr_uarts=8"
save the file and then issue the command
for the changes to take effect
Reboot your machine and check dmesg for the number of serial ports you now have
In my case it looks like this
:/etc/grub.d$ dmesg | grep ttyS
[ 0.709951] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[ 0.710235] 00:07: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[ 0.710400] 0000:05:01.0: ttyS4 at I/O 0xc000 (irq = 19) is a 16550A
[ 0.710477] 0000:05:01.0: ttyS5 at I/O 0xc100 (irq = 19) is a 16550A
[ 0.710551] 0000:05:01.0: ttyS6 at I/O 0xc200 (irq = 19) is a 16550A
[ 0.710626] 0000:05:01.0: ttyS7 at I/O 0xc300 (irq = 19) is a 16550A
[ 0.710700] 0000:05:01.0: ttyS1 at I/O 0xc400 (irq = 19) is a 16550A
[ 0.710775] 0000:05:01.0: ttyS2 at I/O 0xc500 (irq = 19) is a 16550A
You can also check the number and setting of the serial ports using the setserial command for every port
sudo setserial /dev/ttyS7 -a
/dev/ttyS7, Line 7, UART: 16550A, Port: 0xc300, IRQ: 19
Baud_base: 115200, close_delay: 50, divisor: 0
Flags: spd_normal skip_test