How to assert attribute values if there are multiple occurences of attribute

There are probably multiple ways to do this but I would use a Script assertion, something like:

import groovy.util.XmlSlurper

// Get your XML from the appropriate test step response
def xmlText = '''<response>
    <subscriptions-v1>
        <subscription id="1376" status="1">
            <charging-method>1</charging-method>
        </subscription>
        <subscription id="1372" status="1">
            <charging-method>1</charging-method>
        </subscription>
    </subscriptions-v1>
</response>'''

def response = new XmlSlurper().parseText(xmlText)

// Normally you don't need quotes when referencing the result of the parse text (a GPathResult object)
// but in this case we do as the XML element name contains a hyphen (-).
response.'subscriptions-v1'.children().each{ subscription ->

	//Next three lines just for demo purpose
	def subscriptionId = subscription.attributes().get('id') 
	def subscriptionStatus = subscription.attributes().get('status') 
	log.info('subscription id = ' + subscriptionId + ' has the status = ' + subscriptionStatus)

	// Note: attribute will be a string value, so need to assert againat a string.
	assert subscription.attributes().get('status') == '1'
}

More information on the XmlSlurper can be found here.

You can use the XPath Match assertion to check that there are no subscriptions with status other than 1.

XPath expression: count(//subscription[@status!=1])

Expected result: 0