HMRC can usually look back four years, six if you were careless and twenty if the error was deliberate. Which one applies to you decides how many years a disclosure must cover, and how big the bill is.
4 years where reasonable care was taken; 6 years for careless errors; generally 12 years for non-deliberate offshore matters; up to 20 years for deliberate behaviour or failure to notify. Each runs from the end of the tax year (5 April).
A disclosure must cover every year HMRC could assess. Cover too few and HMRC can reopen the settlement; cover too many and you pay tax HMRC could never have collected.