Some of the major hosted feed readers (Inoreader, Feedly, Feedbin) include subscriber count in their user agent. I usually run a filter on requests to the feed links in my access logs to get an idea of how they're changing. Anecdotally, my subcriber count reaches into triple digits with only those counts, but I've never gotten email from readers and generally only get feedback promoting new posts on socials. The counts are about as nebulous as follower counts, which is to say most people probably subscribe/follow and forget.