The description of many Brahma, many Vishnu, many Mahesh comes in the Devi Purana, Bhagwat Purana and, the Bhagwad Gita. Interestingly none of these mention many Krishna in the way others are thus described. This is because creation, preservation and destruction of each of the many universes requires a different process and power but Sri Krishna as Anandamaya is the same everywhere and in everything though hidden behind and limited by the receptacle. Sri Krishna is also regarded as the Immanent Divine in man and hence his multiplicity is of a different kind, as the Supreme Lord dwelling in the heart of every creature.
As to which Sri Krishna did Sri Aurobindo unite with it is clearly mentioned. It was with the earthly personality of Sri Krishna, the Avatar who fused with Sri Aurobindo on the 24th November 1926.
Affectionately,
Alok Da
