I have always understood that a Muslim man is limited to four wives. I recently learned that ibn Saud had 24 wives . How is this possible? Did he receive special dispensation, or did he simply choose to ignore Islamic law in this matter? I am not a Muslim, so I apologize for any mistaken assumptions in this question.
It is completely valid to marry more than four wives (following the known conditions/prerequisites), as is clear in both Quran and Sunnah.
Now what is forbidden is to have more than four wives at any one given time, so if a person already has four, then he would have to divorce one to marry another. So to answer your question about how Bin Saud had many wives, the answer is he divorced many times(as is mentioned in another answer to your question), Source.
The Wikipedia article has a more complete account of the various marriages of Ibn Saud. In particular it notes the following:
Ibn Saud fathered dozens of sons and daughters by his many wives. He had at most only four wives at one time. He divorced and married many times.
Since he never had more than four wives, this was actually compatible with islamic law.