Repression is a function of several things: population of the region, the number and unit size of the garrison units, and if they have any bonuses.

You can easily garrison low population areas with a single unit. I get as much as 2 suppression points for a single militia unit in the Carib islands, for example. So areas with population of <0.1M or thereabouts are actually ideal for building universities. I love my North American holdings for that reason.

In heavily populated areas (many, many millions) each normal unit seems to give only 0.5 suppression points, such as in London, Hindustan, etc. Dragoons, streltsy, genderme, etc, are beneficial in such cases, as they seem to give 1 suppression point per unit.