_ X = 10,000, using 1 Roman Numeral.
Source: Original, but I wouldn't be surprised to find it elsewhere.
1) 1 numeral I=1 2: II=2 3 III=3 4 VIII=8 5 XVIII=18 6 XXVIII=28 7 XXXVIII=38 8 LXXXVIII=88 9 CLXXXVIII=188 10 CCLXXXVIII=288 11 CCCLXXXVIII=388 12 DCCCLXXXVIII=888 13 MDCCCLXXXVIII=1888 14 MMDCCCLXXXVIII=2888 15 MMMDCCCLXXXVIII=3888 _ 16 VMMMDCCCLXXXVIII=8888 __ 17 XVMMMDCCCLXXXVIII=18888 ___ 18 XXVMMMDCCCLXXXVIII=28888 ____ 19 XXXVMMMDCCCLXXXVIII=38888 _____ 20 LXXXVMMMDCCCLXXXVIII=88888 2) Longest MDCCCLXXXVIII = 1888 Largest MCMXCIX = 1999