Hi Sara,
> a hosting server with Portal means using Portal on premise,isn't it ?
Correct.
> which means (as M. Robert said) it does NOT consume credits at all
Not quite, it depends on how you configure Portal. It is possible for Portal to use credits.
> but you've said the opposite ?
To clarify my reply, I meant to say that if you've configured a hosting server with Portal, AND if you've enabled a proxy user (as others have mentioned in their posts) in Portal, then it may consume credits.
For example, Configure Living Atlas content—Portal for ArcGIS (10.5.x) | ArcGIS Enterprise
Hope this helps,