We count any request sent to your Google Tag Manager Server container. It includes all script loads like gtm.js, analytics.js, gtag.js, etc.
To simplify the calculation, you can multiply the number of pageviews in GA by 10. It will be the approximate number of requests. Pro plan includes around 50 000 pageviews/month. Business plan - around 500 000 pageviews/month.
Feel free to use Pricing Calculator to find the best plan for your site.