What is the most common punctuation mark?
Sentences express complete thoughts; as such they need a signal or marker that tells the reader when the thought begins and when it ends. Additionally they need a signal to indicate a pause in the thought to emphasize a section, indicate that a different aspect of the thought is being expressed, separate items in a list, or set a part of the sentence apart for clarity.
Sentences normally begin with capital letters (in English), which signals the beginning of the thought. They end with a question mark, an exclamation point, or a period to tell the reader that the thought is finished. Of the three, the period is most often used; almost every sentence has one.
Commas are used extensively within sentences. Some sentences have a lot of commas while some sentences have none. So although the comma is quite common, it is next to impossible to say whether it is more common than the period.