Air pumps are purely optional. You do not "need" one at all.
I never see them advertised with ratings, but if you pick up a Rena air pump in the box, it tells you clearly how many air stones each air pump will be able to operate at different water depths. If I wanted to run ornaments that amounted to being 3 air stones equivalent and the tank was 60 cm deep, that would mean I needed a particular air pump to operate them. I bought a model 200 to do exactly that on a large tank. When I added another nearby tank and decided to add air to it, I merely added another branching to my air hose, adjusted flow rates and it just kept on doing the trick. The ones rated in liters per hour are less useful to me because I really don't have a feel for how many litres an air stone uses.
