There are just 7 Roman numerals: I, V, X, L, C, D and M.
Unlike most other number systems, the numerals can only be used in particular sequences.
Another rule is that apart from M, you can only have a maximum of 3 of any given numeral in a row.
Here is a quick overview of the 7 Roman numerals and what each is worth.
For larger numbers, we can also use an overline with each Roman numeral which has the effect of multiplying the value by 1000, so:
So XLMM would represent 42,000.