The number should be Monte Cassino 6.
Cross checked with Google Maps and Polish Geoportal

http://mapy.geoportal.gov.pl/imap/

https://www.google.pl/maps/place/Monte+Cassino+6,+30-337+Krak%C3%B3w/@50.0472978,19.9292261,19z/data=!4m2!3m1!1s0x47165b6ff1586a71:0x21cc99bba1d0932b?hl=en