Shri Krishna had eight princely wives, hence he was called Ashtabharya (ashta=eight, bharya=wife). Different scriptures list their names differently. According to Bhagavata puran names of the eight wives in order were Rukmini, Satyabhama, Jambavati, Kalindi, Mitravinda, Nagnajiti, Bhadra and Lakshmana.Apart from these eight, he had 16100 other wives.
So why did the lord have so many wives ? The story goes..
The son of mother earth was called Bhaumasura (Bhumi = Earth and Asura = Demon).The demon king Bhaumasura had kidnapped 16,100 women and held them in his captivity in his capital Pragjyotisha. Bhaumasura had stolen the ear-rings of Aditi (daughter of brahma, first nascent being), her sons demigods were unable to fight against Bhaumasura. Upon their request, Shri Krishna along with his wife Sathyabhama attacked Pragjyotisha on his vehicle Garuda (eagle) and fought a fierce battle against the demons, in that war, many powerful demons were slayed by Shri Krishna. Most important of those were Mura and Bhaumasur. Bhaumasura had the curse of being killed by his own mother. Sathyabhama is considered the incarnation of mother earth, thus she killed Bhaumasura.
After killing Bhaumasura, Shri Krishna released 16,100 women that he had in captivity. When Shri Krishna asked them to return to their houses, they refused. They were aware that the society of that age would not take back those who were taken by another man. So they were left with nowhere to go. When Shri Krishna asked them what they wanted to do, they all wanted Shri Krishna to marry them. Shri Krishna married them all at the same auspicious time assuming that many bodies, so they could gain status of married women and live in the society with honour again. Krishna constructed them each a palace with huge gardens full of colourful flowers and birds. It was impossible to live with each of them so he created 16100 forms of himself so that he could treat them all in the same manner as his eight wives.
Shri Krishna is the supreme god himself. He is the only sanctuary of the manifested material world. He knows everything and is all that is knowable. He is above the material modes. With limitless forms, he pervades the whole cosmos. He is immeasurable.