For each policy definition id reference each input variable created above using ${var.variableName}

AzurePolicySetResource.tf

resource "azurerm_policy_set_definition" "tag_governance" {
  name         = "tag_governance"
  policy_type  = "Custom"
  display_name = "Tag Governance"
  description  = "Contains common Tag Governance policies"
  metadata = <<METADATA
    {
    "category": "${var.policyset_definition_category}"
    }
METADATA
  policy_definitions = <<POLICY_DEFINITIONS
    [
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_0}"
        },
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_1}"
        },
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_2}"
        },     
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_3}"
        },
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_4}"
        },
        {
            "policyDefinitionId": "${var.addTagToRG_policy_id_5}"
        }
    ]
POLICY_DEFINITIONS
}