B isn't a number, so asking what it looks like "in Roman numerals" is nonsense.
In the Greek numbering system, the letter beta represents the number 2.
The letter B is not used in Roman numerals. Roman numerals use letters I, V, X, L, C, D, and M to represent numbers.
12 = XII in Roman numerals
18 in roman numerals looks like XVIII
The Roman numerals are XLV.
300 would be represented by 'CCC' in roman numerals.
MDCCCXCV
MMDCCCLXXXVI
MCMLXXXV
MCMLXV
XXI
DCLXVI
MCMLII
MCMLXXXIX