Turtle checksums

Here is a Turtle Art program that calculates the checksum of a South American 7 digit identity number. Source. You can get the algorithm as a spreadsheet

Challenges:
How many ID numbers share a checksum?
Find 2 ID numbers with the same checksum, not by trial and error
The 'key' is 2987634, what restrictions are there on the key that could be used?
What is the effect of having the digit 0 in the key?