How do I start rows with 0 (not 1) or not count header rows in Numbers?
So, this is easy to solve in Excel...  I don't have access to Numbers at the moment, but I'll give you the solution I'm aware of and you can confirm whether it works in Numbers like it does Excel.

You can create a column with the following formula:

=Row() - 1

This will always give you one less than the "default" row number, which is what you are looking for.   It also won't be affected by sorting.

A complete reference of functions that are available in Numbers can be found HERE, including the ROW function.