There has never been a Pope John XX, because the 20th pope of this name, formerly Petrus Hispanus, when elected Pope in 1276, decided to skip the number XX and to be counted as John XXI instead. He wanted to correct what in his time was believed to be an error in the counting of his predecessors John XV to XIX.
Confusion in numbering Popes John results from an error in the textual transmission of the entry on John XIV (983/984) in the Liber Pontificalis. This entry originally specified not only the duration of his pontificate ("VIII mens." = eight months), but also the duration of his ensuing imprisonment by Antipope Boniface VII, "per IV menses" ("for four months"). In the 11th century, some time after the pontificate of John XIX, this entry on John XIV was misread to be referring to two different popes John, the first reigning for eight months and directly succeeded by another John reigning for four months:
In distinguishing these two Johns, the second one came to be numbered as "Iohannes XIV. bis" ("John XIV the second") and it was confused with a historic character, the cardinal deacon John son of Robert, who opposed Boniface VII after John XIV's death. Given the fact that the following Popes John, from John XV (985–996) until John XIX (1024–1034), seemed to have neglected the existence of John XIV "bis", Petrus Hispanus meant to correct this error by electing for himself the name John XXI.