as I understood, if I chose Nina in the end I will have a chance to choose one or two more girls and that's it? or will there be only one
You can have two, at max three girls in a mini-harem.
If you play your cards right, that is.
That mini-harem is one option. The other is regular relationship.
To be as exact as possible 4 or so girls will be OK with other girls in the relationship as well in general. Some of them are absolutely not OK when a specific other girl is already in that relationship as they hate each other. Others are only OK with it when their best friend is in it already.
Whether or not Nina is one of the possible candidates we don't know and as Samu said the dev refuses to "spoil it".
As I said previously Nina might have suggested a harem in an attempt to gain protection after more or less insulting a powerful family. If she actually would be OK with that thought is another issue entirely.
We only know about one girl for sure that she won't be OK with that thought.
And that is Yandere-with-tentacles/snakes Lilly.
For almost all others (not very likely that the dean is interested at all) it is still unclear.
And it likely remains unclear for a long time.