The Roman numeral MM is 2,000. M is the Roman numeral for 1,000. Roman numerals are read using the additive principle, which simply means that they are added together. So two Ms would be 1,000 + 1,000, which equals 2,000.
Roman numerals are also written highest to lowest, with the highest numbers on the left and the lowest on the right. Roman numerals are also written using the smallest amount of symbols possible. Therefore, even though CCCCCCCCCCCCCCCCCCCC would also be 2,000, since C is the Roman numeral symbol for 100, it is an incorrect way to write 2,000, because it is not the smallest number of characters in which 2,000 can be expressed.