How can I find out how many subscribers I have on my podcast?

You can get some data through Apple's Podcast Analytics - please see

https://itunespartner.apple.com/podcasts/articles/check-your-podcast-performance-3071

and

https://itunespartner.apple.com/podcasts/whats-new/100002358

However this information is partial. It checks downloads of episodes, and only by people using recent apps and who are willing to share data. Note that Apple says:

'Note that Podcast Analytics begins gathering data as soon as users play content in the Podcasts app (iOS 11 or later) or on iTunes (12.7 or later on either macOS and Windows). The data is aggregated and no personally identifiable information for any customer is shown.'

You cannot determine exactly how many subscribers you have because even if you set your server to record requests each user checks the feed at regular intervals