To the question of whether Buddhists can keep more than one wife, the direct answer is not available in the Buddhas teaching, because as mentioned earlier, the Buddha did not lay down any religious laws with regard to married life although he has given valuable advice on how to lead a respectable married life.
Tradition, culture and the way of life as recognized by the majority of a particular country must also be considered when we practice certain things pertaining to our lives. Some religions say that a man can have only one wife whilst others say a man can have more than one wife.
Although the Buddha did not mention anything regarding the number of wives a man could have, he explicitly mentioned in His discourses that should a married man go to another woman out of wedlock, that could become the cause of his own downfall and he would have to face numerous other problems and disturbances.
The Buddhas way of teaching is just to explain the situation and the consequences. People can think for themselves as to why certain things are good and certain things are bad. The Buddha did not lay down rules about how many wives a man should or should not have which people are forced to follow. However, if the laws of a country stipulate that marriages must be monogamous, then such laws must be complied with, because the Buddha was explicit about His followers respecting the laws of a country, if those laws were beneficial to all.