As a native English speaker (and former English teacher), hope this helps:
> getInstances(); // ok, returns an array of instances
Correct.
> getInstanceId(); // ok, returns an id
Correct.
> getInstancesId(); // ??, returns an array of id
Unusual, but would return a single ID referring to a collection of instances.
> get InstancesIds(); // ?? returns an array of id
Correct.
Though more common would be getInstanceIds() since you don't need to pluralize twice. E.g. we don't say "blues shoes", we say "blue shoes", so these aren't "instances ID's" but "instance ID's".
However, in naming functions or variables you'll sometimes see plurals repeated for clarity. E.g. getInstanceIds() is ambiguous because it could refer to the ID's of multiple instances, or multiple ID's of the same instance. While getInstancesIds() makes it clearer that it's the ID's of multiple instances, even though it's not actually grammatically correct.
> getIdsOfInstances(); // ?? returns an array of id
Same as getInstanceIds(). Both are correct, though getInstanceIds() would probably be more common -- most people wouldn't put in the extra word "of" simply because it's an extra word, but sometimes it might be clearer for consistency with other function names.
> getAllInstancesId(); // ?? returns an array of id
It would mean the ID for the collection of all instances, though that would seem very unusual. Perhaps if there were a global account ID for a service or API you used.
> getAllInstancesIds(); // ?? returns an array of id
Sams as getInstanceIds(), but "All" presumably means it doesn't include a parameter to filter -- so I'd assume this implied the existence of another function such as getCollectionInstanceIds() or similar that it was contrasted against.
> How do I conjugate that ? And what about the possessive `s`?
Obviously you can't or shouldn't use apostrophes in variable names, so people generally try to avoid the possessive s. That's why we generally change nouns to noun adjectives -- instead of "computer's ID" (computer = noun) we use "computer ID" (computer = noun adjective).