AWS Opensearch multiple instances reservation

Hello.

which cluster would the reservation apply to?

This is just my guess, but I think it will be randomly assigned between clusters.
I recommend that you open a case with AWS Support under "Account and billing" to determine the applicable order.
Inquiries under "Account and billing" can be made free of charge.
https://docs.aws.amazon.com/awssupport/latest/user/case-management.html

If I delete one of the clusters in the future, would the reservation automatically apply to the remaining cluster?

Yes, I believe that when you delete the cluster, it will be assigned to Opensearch, which is the same instance type as the Reserved Instance.
https://docs.aws.amazon.com/opensearch-service/latest/developerguide/ri.html

RIs are not flexible; they only apply to the exact instance type that you reserve. For example, a reservation for eight c5.2xlarge.search instances does not apply to sixteen c5.xlarge.search instances or four c5.4xlarge.search instances. For full details, see Amazon OpenSearch Service pricing and FAQ.