The min number of compute instance in a instance group cannot be 0.
It give me an error if I put a 0 as a min or max when editing an instance group
I do not see a time where you ever would want 0. The point in the autoscaling / instance groups is to help maintain availability and scalability of the infrastructure. By setting it to 0 you are saying, I really don’t care if everything is unavailable and I do not even care if you scale one in.
If you just want the instances set to 0 in the instance groups you will need to delete the Auto-scaling section, that will let you select to have 0 instances – but i really cannot see why you ever would want that, however it could be better to use an unmanaged instance group then as previously it sounds like you are wanting an unmanaged managed instance group..
You have to delete the autoscaling configuration first. This will remove the minimum and maximum limit on the instance group. After that just update the "Number of instances" on the page to 0 and it should work. Hope this helps.