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That number is 4 digits short, if you registered your copy, you can get your serial number from your Adobe Store account.
Registration and activation are two unrelated things.
If you de-activate the program via the Help menu before reformatting you just re-activate the new installation – no numbers required.
If it won't activate, call the special number on the activation screen (not Customer Service) and explain – they will reset your counter.
If you've lost your serial number you won't even be able to re-install without getting it from your store account.
Good tip Bob – I'd forgotten that one!