6 lantai. Karena lantai dasar tidak dihitung sebagai 'tingkat'.

In my opinion, mostly Indonesian count in the ground as "tingkat", so it would be tingkat 1 : ground tingkat 2 : ground + 1 tingkat diatas tingkat 3 : groung + 2 tingkat diatas and so on.

In most areas in Indonesia, "bertingkat lima" definitely means "(have) five stories". The ground floor is considered as a "tingkat". However, "tingkat/lantai lima" can mean fifth or sixth storey/floor. It depends if the ground floor is considered as a "tingkat" in a building (some hotels do this).