Accurately counting how many people are following you is impossible. You will always see different numbers in each client.

Why is this, you may ask? It is because of how follow lists are stored on the relays.

When you follow someone, their npub is added to a list that is posted to whatever relays you specify. This makes it very easy for a client to display how many people YOU follow. It just has to find your most recently updated follow list and add up the number of npubs listed there. Done.

When someone else follows you, nothing different happens. Their follow list is updated to add your npub. This means there is no unified list stored anywhere with who is following you. Instead, the client has to ask as many relays as possible, "Please send me any follower list that has FLASH's npub listed on it," and count how many it receives after weeding out the duplicates.

What if there are relays that are down? What if not all the relays it asks are currently reachable? Or even more likely, what if the client isn't asking every single relay that has a follow list with your npub on it? Well, then your follow count will be less than the actual amount.

Follower counts should always be considered an approximation at best.