Requisite numer of members (for ecumenical status)
“The number of bishops present required to constitute an Ecumenical
council cannot be strictly defined, nor need it be so deigned, for ecumenicity chiefly depends on co-operation with the head of the Church, and only secondarily on the number of co-operators. It is physically impossible to bring together all the bishops of the world, nor is there any standard by which to determine even an approximate number, or proportion, of prelates necessary to secure ecumenicity. All should be
invited, no one should be debarred, a somewhat considerable number of representatives of the several provinces and countries should be actually present; this may be laid down as a practicable theory. But the ancient Church did not conform to this theory. As a rule only the patriarchs and metropolitans received a direct summons to appear with a certain number of their suffragans. At Ephesus and Chalcedon the time between the convocation and the meeting of the
council was too short to allow of the Western bishops being
invited. As a rule, but very few Western bishops were personally present at any of the first eight general synods. Occasionally, e.g. at the sixth, their absence was remedied by sending deputies with precise instructions arrived at in a previous
council held in the West. What gives those Eastern synods their Ecumenical character is the co-operation of the pope as head of the universal, and, especially, of the Western, Church. This circumstance, so remarkably prominent in the Councils of Ephesus and Chalcedon, affords the best proof that, in the sense of the Church, the essential constituent element of ecumenicity is less the proportion of bishops present to bishops absent, than the organic connection of the
council with the head of the Church.”
newadvent.org/cathen/04423f.htm