It is not possible to pull reports in Megaphone on the number of subscribers to your show. Most applications and listening platforms do not release total subscriber numbers publicly. Apple's Podcasts Connect will show some subscriber reporting, however the platform is in beta, only shows information from recent Apple softwares, and does not include a complete picture of subscribers.
In addition to the lack of publicly available metrics, the podcast and advertising industry at large relies on downloads per episodes rather than subscribers per show. When discussing the success of your podcast - especially for gaining advertisers - you should always use the industry standard of downloads rather than subscribers.