The genitive, in this case, functions like the prepositional phrase "of ..." in English, so the object of the preposition is independent with respect to number. Although the genitive describes or qualifies similar to the way that adjectives do, there is no requirement for numerical agreement as there is with adjectives.
For example, in English, we might speak of a house of cards, with house being singular and cards being plural. The number of cards is not in any way determined by the number of houses.
In the same way, pubis in the singular, is not affected by mons being singular or plural.
Here's some more information on the genitive:
GENITIVE WITH NOUNS
342. A noun used to limit or define another, and not meaning the same person or thing, is put in the Genitive.
This relation is most frequently expressed in English by the
preposition of, sometimes by the English genitive (or possessive)
librī Cicerōnis, the books of Cicero, or Cicero's books.
inimīcī Caesaris, Cæsar's enemies, or the enemies of Cæsar.
talentum aurī, a talent of gold.
vir summae virtūtis, a man of the greatest courage.