I still don't get the meaning of the preposition po
Could you give me some examples where it is used ?
That preposition could have been taken from Bulgarian, which has the same preposition:
Every time 4 persons sit at the (3) tables = 12 persons. 4 Persons are sitting at each table.
I have taken this weird example by choice: po here functions more like an adverb, and even Zamenhof also used the accusative with po.
There is a corresponding per in English - at the other place. So po means each time, by. German has jeweils, Dutch telkens.
It expresses a quantitative unit repeated: each time.
La intervjuoj prenis po du personoj en ĉiuj ĉambroj.
The interviews each took two persons in every room.
Ŝi veturis po ducent kilometrojn en horo.
She drove two hundred kilometres per hour.
In fact, one could (semantically) combine po with a preposition:
La intervjuoj okazis kun po du personoj en ĉiuj ĉambroj.
The interviews happened with two persons each in the rooms / per room.
I would personally see it as an adverb before a numeral.