"a certain quantity" is a correct and usable phrase in written English. It is typically used as a way of referring to an unspecific amount of something, such as in the sentence "She bought a certain quantity of apples at the store.".
The first part stipulates that all states receive a certain quantity regardless of their population or risk status.
"The continual cracking of your feet on the road makes a certain quantity of road come up into you" (The Third Policeman).
But when you're putting together an institution of a certain type of manners and morals, it makes sense to go in for a certain quantity of homogeneity.
She represents "a real human being with a heart, with feelings, with sympathies and with emotions only as a bank-note represents a certain quantity of gold".
When the mineral does not contain much barium, a certain quantity of barium salt is added in order to carry away the radium.
A futures contract is an agreement to deliver or receive a certain quantity of a commodity at an agreed price at some stated time in the future.
Towns typically meet these kinds of obligations by hinging their approval of new developments on the applicants' promises either to bankroll or build a certain quantity of "affordable" housing.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union