Number generation for a table depend on sys_number and sys_number_counter table.
From sys_number table , the prefix of the number for that specific table will be decided.
sys_number_counter table will show the last number generated for that specific table , so that when creating a new record , the number is generated based on the number in sys_number_counter by incrementing it to 1.
For question -> Do we have separate Number maintenance theory for Domain separated instance , the answer is no.
Number is not generated differently nor based on the domain we are logged in.
Ex : INC000050 is created in Domain A ,
When you try to create incident in Domain B , it starts with INC000051 and the record created in Domain B will not be accessible to Domain A if they are siblings.
But if the incident is created in Parent Domain, Childs will have access to that record.