aws ec2 describe-reserved-instances
{
    "ReservedInstances": [
        {
            "Duration": 57024000,
            "End": "2026-02-12T16:11:46+00:00",
            "FixedPrice": 0.0,
            "InstanceCount": 3,
            "InstanceType": "r5a.xlarge",
            "ProductDescription": "Linux/UNIX",
            "ReservedInstancesId": "c5851026-2b6f-4d5c-99ba-946146221ac0",
            "Start": "2024-04-23T16:11:47.169000+00:00",
            "State": "active",
            "UsagePrice": 0.0,
            "CurrencyCode": "USD",
            "InstanceTenancy": "default",
            "OfferingClass": "standard",
            "OfferingType": "No Upfront",
            "RecurringCharges": [
                {
                    "Amount": 0.099,
                    "Frequency": "Hourly"
                }
            ],
            "Scope": "Region"
        }
    ]
}

aws ec2 create-tags --resources c5851026-2b6f-4d5c-99ba-946146221ac0 --tags Key=project,Value=200029

aws ec2 describe-reserved-instances                                                                 
{
    "ReservedInstances": [
        {
            "Duration": 57024000,
            "End": "2026-02-12T16:11:46+00:00",
            "FixedPrice": 0.0,
            "InstanceCount": 3,
            "InstanceType": "r5a.xlarge",
            "ProductDescription": "Linux/UNIX",
            "ReservedInstancesId": "c5851026-2b6f-4d5c-99ba-946146221ac0",
            "Start": "2024-04-23T16:11:47.169000+00:00",
            "State": "active",
            "UsagePrice": 0.0,
            "CurrencyCode": "USD",
            "InstanceTenancy": "default",
            "OfferingClass": "standard",
            "OfferingType": "No Upfront",
            "RecurringCharges": [
                {
                    "Amount": 0.099,
                    "Frequency": "Hourly"
                }
            ],
            "Scope": "Region",
            "Tags": [
                {
                    "Key": "project",
                    "Value": "200029"
                }
            ]
        }
    ]
}

Magari esisterà anche un altro modo, ma questo ha funzionato subito taggando le risorse