فَيُصَلِّيَ بِهِمْ إِمَامُهُمْ خَمْسَ تَرْوِيحَاتٍ
Translation: Then their imam leads them in five tarwihahs.
One tarwihah comprises four rakahs. Therefore five tarwihahs are twenty rakahs in the established Hanafi arrangement. The three rakahs of witr are separate, not included in the twenty.
The arithmetic is 5 × 4 = 20, or ten pairs of two rakahs.