From Dehradun, a full circuit is a week-ish trip if you want to stay human. Two-dham splits (Yamunotri–Gangotri, Kedarnath–Badrinath) are kinder.
Fleet
4 pax: Innova or XUV700. 6–7: Scorpio-N. 8–12: Tempo 12s. Urbania if you want coach seats without a bus.