Pope John XX (Pope John the 20th) doesn't actually exist. There was a numbering mistake made in the 11th century because historians at the time thought that there was a Pope John before Pope John XV (Pope John the 15th). So they numbered Pope John XV as Pope John XVI (Pope John the 16th). The error originated during the transcription of the "Liber Pontificalis" (Book of the Popes), a collection of pope biographies. The error applied to Pope John XV to Pope John XIX (Pope John the 15th - 19th). The mistake was realized later and the Popes were renumbered.
However, it is important to note that some historical sources were not corrected and may refer to Pope John the 20th as the 21st and so forth. What made matters more confusing were the over-correcting by some sources and the presence of antipopes, individuals who claimed to be pope but were not.
Considering the numbering correction and the antipopes, there have only been 21 valid Pope Johns as of 2015.
More about the Pope and numbering: