The Vatican contains 4.54 living Popes per square kilometer, or .0454 Popes per hectare, or 11.76 Popes per square mile, or 0.018 Popes per acre, or 2 Popes per Acre. To include non-living Popes, multiply by anywhere from 69.5 to 70.5: 315.53-320.07 per square kilometer, or 3.155-3.2 per hectare, or 817.32-829.08 per square mile, or 1.251-1.269 per acre, or 139-141 per Acre.
Source
1,
2, and
3