Most developers think count and for_each are just two ways to repeat resources.
They are not.
Terraform does not track “resources.”
It tracks addresses in state.
With count, the address is positional.aws_vpc.example[0] is tied to an index.
Remove the first item in a list and every index shifts.
Shifted index means new address.
New address means destroy and recreate.
With for_each, the address is keyed.aws_vpc.example["prod"] is tied to a label.
Reordering does not matter.
Stable keys preserve identity.
The real design question is this:
Are your instances interchangeable slots,
or named roles that must persist?
Choose the wrong identity model and a refactor becomes an outage.