https://www.catholicculture.org/culture/library/dictionary/index.cfm?id=31865
ANTIPOPES OF THE WESTERN SCHISM:
CLEMENT VII (Robert of Geneva): September 20 (October 31), 1378 to September 16, 1394.
BENEDICT XII (Pedro de Luna): Aragon; September 28 (October 11), 1394 to May 23, 1423.
ALEXANDER V (Pietro Filargo): Crete; June 26 (July 7), 1409 to May 3, 1410.
JOHN XXIII (Baldassare Cossa): Naples; May 17 (25), 1410 to May 29, 1415.
FELIX V (Amadeus, Duke of Savoy): Savoy; November 5, 1439 (July 24, 1440) to April 7, 1449; d. 1451.
The Church has flip-flopped on the issue in the past. The Pisan popes were widely regarded as valid popes until the 20th century and, as Roscoe pointed out, this is reflected in papal regnal numbering too. For example, Pope Alexander VI clearly recognised Alexander V as his legitimate predecessor.
It was only with Pope John XXIII(the 20th century one) that the official line on this appeared to change, as he ignored the 15th century John XXIII in his regnal numbering, saying there were only 22
certainly valid Johns before him(so
still not saying the other John XXIII was certainly invalid).