How do I get the computer's SNID number from Ubuntu 20.04?

You can find the serial number ID by enter the following in a terminal:

sudo dmidecode -s string

where string is replaced by any of a number of parameters...

bios-vendor
bios-version
bios-release-date
system-manufacturer
system-product-name
system-version
system-serial-number
system-uuid
baseboard-manufacturer
baseboard-product-name
baseboard-version
baseboard-serial-number
baseboard-asset-tag
chassis-manufacturer
chassis-type
chassis-version
chassis-serial-number
chassis-asset-tag

So in your example, to find the SNID you would use:

sudo dmidecode -s system-serial-number

reference