Followers can only ever really be estimated. To count your followers, you basically need to crawl profiles from the known network (all the relays your clients knows of) and see if they are following you. But of course, some of their relays might be unknown to you. Or worse, different relays might have different versions of their profiles. So it's not an easy number to get...
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You cannot recover followers, but you can recover the people you follow. Some apps have messed up my follow list. So at some point I was longer following you. Less frequent lately, but that can factor in of course. That's why it's useful to back up your stuff once in a while with things like https://metadata.nostr.com
Hope it helps!
And download the backup