Namesakes
Bishops of Rome and the Popes:
23 popes, 2 antipopes
John I (a.k.a. Ioannes) 523-526 
John II (a.k.a. Ioannes) 533-535
John III (a.k.a. Ioannes) 561-574
John IV (a.k.a. Ioannes) 640-642
John V (a.k.a. Ioannes) 685-686
John VI (a.k.a. Ioannes) 701-705
John VII (a.k.a. Ioannes) 705-707
John VIII (a.k.a. Ioannes) 872-882
John IX (a.k.a. Ioannes) 898-900
John X (a.k.a. Ioannes) 914-928
John XI (a.k.a. Ioannes) 931-935
John XII (a.k.a. Ioannes) 955-964
John XIII (a.k.a. Ioannes) 965-972
John XIV (a.k.a. Ioannes) 983-984
John XV (a.k.a. Ioannes) 985-996
John (a.k.a. Ioannes) 997-998
John XVII (a.k.a. Ioannes) 1003
John XVIII (a.k.a. Ioannes) 1003-1009
John XIX (a.k.a. Ioannes) 1024-1032
John XXI (a.k.a. Ioannes) 1276-1277
John XXII (a.k.a. Ioannes) 1316-1334
John (a.k.a. Ioannes) 1410-1415
John XXIII (a.k.a. Ioannes) 1958-1963
John Paul I (a.k.a. Ioannes) 1978
John Paul II (a.k.a. Ioannes) 1978-2005
John I (a.k.a. Iôannês) 496-505
John II (a.k.a. Iôannês) 505-516
John III (a.k.a. Iôannês) 680-689
John IV (a.k.a. Iôannês) 777-799
John V (a.k.a. Iôannês) 1147-1166
John VI (a.k.a. Iôannês) 1189-1216
John VII (a.k.a. Iôannês) 1262-1269; 1271-1293
John VIII (a.k.a. Iôannês) 1300-1320
John IX (a.k.a. Iôannês) 1320-1327
John X (a.k.a. Iôannês) 1363-1369
John XI (a.k.a. Iôannês) 1427-1452
John XII (a.k.a. Iôannês) 1480-1483
John XIII (a.k.a. Iôannês) 1484-1524
John XIV (a.k.a. Iôannês) 1571-1586
John XV (a.k.a. Iôannês) 1619-1629
John XVI (a.k.a. Iôannês) 1676-1718
John XVII (a.k.a. Iôannês) 1727-1745
John XVIII (a.k.a. Iôannês) 1769-1796
John XIX (a.k.a. Iôannês) 1928-1942
Roman and Byzantine Emperors:
8 emperors
Emperor Johannes Tzimisces (a.k.a. Ioannes) 969-976
Emperor Johannes Comnenus (a.k.a. Ioannes) 1118-1143
Emperor Johannes Ducas Vatatzes (a.k.a. Ioannes) 1221-1254
Emperor Johannes Ducas Lascaris (a.k.a. Ioannes) 1258-1261
Emperor Johannes Cantacuzenus (a.k.a. Ioannes) 1341-1354
Emperor Johannes Palaeologus (a.k.a. Ioannes) 1390
Emperor Johannes Palaeologus; John V Palaeologus (a.k.a. Ioannes) 1341-1376; 1379-1390; 1390-1391
Emperor Johannes Palaeologus (a.k.a. Ioannes) 1425-1448
Saints:
4 saints
Saint Tornike Eristavi (a.k.a. Ioannes) c. 920-985
Saint John Ducas Vatatzes (a.k.a. Ioannes) 1192-1254
Saint John XXIII (a.k.a. Ioannes) 1881-1963
Saint John Paul II (a.k.a. Ioannes) 1920-2005