In the phrase
c'eravamo tanto amati aren't we using congiuntivo passato ? In such a case we use past participle
amato regardless of gender and number.
c'eravamo tanto amati
is in the reflexive voice. Precisely it is what is called a "reciprocal reflexive" (indicating that the action is done by two or more subjects, acting on each other). In Italian, verbs in the reflexive voice want the auxiliary verb essere, and therefore all participles in composed tenses need to agree in number and gender with the subject.
In this particular case the subject is an implicit noi (i.e. "we"), and so the past participle is correctly in the masculine plural (masculine being the default gender in absence of other information).
To also add on the choice of the tense, this is not congiuntivo passato, rather (indicativo) trapassato prossimo. If the sentence were in congiuntivo passato it would be
[Che] ci siamo tanto amati
and in this particular case (1st person plural) it would be indistinguishable from the (indicativo) passato prossimo.