I doubt if the Romans ever needed to use such a large number. M (1000) was the biggest numeral they used and so 10 million would be a string of 10000 Ms. Some sources say that a numeral with a bar (horizontal line) across its top represented that numeral multiplied by a thousand, so it could be a string of 10 Ms with a bar across the top, ie 100000 x 10. Searching google throws up some interesting sites.