Before I moved to Australia, I used to live in a マンション in Kyoto! That had 5 floors up and there were 5 rooms on each floor. That sounds good, doesn’t it?
However, the space I occupied was about 3.5 meters wide and 5 meters long. It had a kitchen, a bathroom with a toilet and a shower over bath, and a living space with a small built-in called 押し入れ （おしいれ）.
マンション in Japanese is the same as a “unit” or “flat” in Australia. Although the building is big like a “mansion” in English, it is divided into many units and people occupy their own unit only.
The マンション I was living in was actually called ワンルームマンション which best translates as a “studio” in Australia. Many single people in Japan live in a room like that. Other マンション are usually described like 2DK, 3LDK, etc. A 2DK has 2 rooms and 1 dining kitchen while a 3LDK has 3 rooms, a living room and a dining kitchen.
So even if your Japanese friend invites you to his/her mansion, don’t expect a castle like building you see in movies.