"The Numeral Letters that compose the Roman Character are in Number seven, viz., I, V, X, L, C, D, M." (Chambers) but, here's a site that lists additional characters for numbers such as 40, 80, 250, 300, 400, etc. (but no authority is given for their usage)
-joe (i am not a skeptic) friday

http://palimpsest.stanford.edu/don/dt/dt2896.html