How does AdMob count impressions in ListView

Question

I have a ListView in which I show an add for each position % 5. So for example if I have 16 items in a list, then I show an ad 4 times. The way AdMob works is you have to declare the ad unit inside the xml as

<com.google.android.gms.ads.AdView
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    ads:adSize="BANNER"
    ads:adUnitId="@string/ad_unit_banner"/>

My question is this: If a user scrolls the full length of my ListView, does AdMob count it as four impressions or as one impression since the exact same ad_unit_banner is repeated throughout the list? Does it matter if I set the add to change every 30 seconds vs every 60 seconds?