According to Official Guinness Records,
The most goals scored by a football (soccer) goalkeeper in one season is 21, achieved by Rogério Ceni (Brazil), playing for playing for São Paulo Futebol Clube, from 23 January to 14 December 2005.Football seasons in Brazil run alongside the calendar year. The last goal scored in that season by Rogério Ceni was during the FIFA Clubs World Cup, last tournament of the year.
For a complete list of 2005 records, please visit 2005 Guinness Records in Brazil.