instance-count.py

import boto3

ec2 = boto3.client('ec2');
cloudwatch = boto3.client('cloudwatch');

def lambda_handler(event, context):
    count = 0
    response = ec2.describe_instances()
    for r in response['Reservations']:
        count += len(r['Instances'])

    cloudwatch.put_metric_data(
        Namespace='EC2',
        MetricData=[
            {
                'MetricName': 'InstanceCount',
                'StatisticValues': {
                    'SampleCount': count,
                    'Sum': count,
                    'Minimum': count,
                    'Maximum': count
                },
                'Unit': 'Count'
            },
        ]
    )