Unfortunately, we are unable to tell you the exact number of subscribers to your playlists. Here's why: A Listen Later playlist is like a podcast, which is distributed via RSS. There's not way to figure out the exact number of subscribers to a specific podcast due to the limitation of the RSS-based content distribution mechanism.
We try our best to provide some insights to playlist creators. You can visit the INSIGHTS tab on your playlist page and see most recent access logs to the RSS feed:
These access logs can give you some basic idea what podcast apps are used to listen to your playlist. A few podcast apps tell you the number of subscribers to your playlist on their platform, e.g., Overcast/1.0 Podcast Sync (4 subscribers; feed-id=980437; +http://overcast.fm/). But most apps don't disclose this type of information.