You certainly won't be asked to fill one in for every year worked for the last 20 years .
As far as I know , around 10 % of paye workers are sent a form 12 to be filled in.
This may be activated by a change in your normal tax credits , like maybe , company shares , dividends , capital gains , chargeable assets etc .
Its easy enough to fill in , however always answer every question asked and tick all necessary boxes , and enter zero's whereever the question dont apply to your situation .