My home has 2 floor if you enter from one side and 4 from the other side. Due to being on a hill. I wonder how to convince osm of that :D

There's exactly that example on the official OSM wiki page [1]. It's very useful to read their wiki while adding things to the map!

OSM and all related projects thrive on users' contributions -- be the change you want to see in the world and start contributing today :)

[1] https://wiki.openstreetmap.org/wiki/Key:building:levels

You can tag it accordingly with this guide https://wiki.openstreetmap.org/wiki/Key:building:levels

Happy mapping!