Is that only very few app really use mult cores power? I google this question but there is almost no discussion. So only very few apps really use more than one core?
It's a pretty subjective question, since no one really knows how many apps do or don't do any given thing. You'd have to carefully analyse each app to find out how much use it can make of multiple cores.

Likely, more apps than you realise are doing this. First off, Android makes it very simple for app authors to work in multiple threads, with helpers like AsyncTask. App developers are encouraged to do most operations in background threads, such as loading bitmaps from storage, transferring data over the network, or long computations. Running those on a second core keeps the app more responsive while the operation's going on.

If the app does anything with GL (most graphics-heavy apps and almost all games do), then it's certainly using two cores. How much use it's making of that freedom to parallelise is different for each app, but again, it lets the app do long-running operations while still responding to you.

On top of this, even if the app you're using right now is only running in a single thread on one core, anything else Android is doing in the background will be running on another core, which again lets your app stay responsive. This might include activities like drawing the system UI, running background services from other apps to check your mail &amp;c., features like "Smart Stay" on Samsung phones, listening for the "OK Google" hotword, and more.

So in short, while we can discuss how much use any given app makes of the power available to it, a sweeping statement like, "very few apps really use more than one core," sounds like the kind of thing an uninformed salesman would say to try to impress you.