Deploy multiple resource grops in Azure

azureDeployMultipleResourceGroups.json

{
	"$schema": "https://schema.management.azure.com/schemas/2018-05-01/subscriptionDeploymentTemplate.json#",
	"contentVersion": "1.0.0.0",
	"parameters": {
		"rgNames": {
			"type": "array"
		},
		"rgLocation": {
			"type": "string"
		}
	},
	"variables": {},
	"resources": [
		{
			"type": "Microsoft.Resources/resourceGroups",
			"apiVersion": "2018-05-01",
			"location": "[parameters('rgLocation')]",
			"name": "[parameters('rgNames')[copyIndex()]]",
			"copy": {
				"name": "rgCopy",
				"count": "[length(parameters('rgNames'))]"
			}
		}
	]
}

resourceGroupParameterFile.json

{
	"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentParameters.json#",
	"contentVersion": "1.0.0.0",
	"parameters": {
		"rgNames": {
			"value": [
				"group1",
				"group2",
				"group3",
				"group4"
			]
		},
		"rgLocation": {
			"value": "eastus"
		}
	}
}

As noted above, the problem is solved by providing the proper command syntax. The correct command invocation to deploy resources at subscription level is as follows: new-AzSubscriptionDeployment -Name <name-of-your-choice> -Location <locationCode> -TemplateFile <path-to-file> -TemplateParameterFile <path-to-another-file>

On a side note, it's curious that the template file is still being read with the -TemplateUri command, as shown by the proper expectation of rgNames parameter value, but the parameter file is not being read with the -TemplateParameterUri, parameter.