The numerical system of ancient Rome is utterly useless for mathematics. But some have retained it, because Latin numbers make a good alternate numbering system for LABELLING purposes, like numbering the chapters in a book. Here's how the numerals run.
The numbers I, II and III are simple enough, they're 1, 2, and 3. Then comes the foremost peculiarity of Latin numerals. Certain numbers are treated as "something MINUS ONE." The Roman numeral for five is V; thus, four is five minus one, written as IV. On the other side of five, more ones are added; so, 6, 7 and 8 are VI, VII and VIII. Then the "minus one" is used again for the number nine: nine and ten are IX and X.
Counting eleven through thirty goes as follows:
XI, XII, XIII, XIV, XV, XVI, XVII, XVIII, XIX, XX, XXI, XXII, XXIII, XXIV, XXV, XXVI, XXVII, XXVIII, XXIX, XXX.
Fifty is L; one hundred is C; and one thousand is M. The same peculiarity continues to apply.
Thus, the current year 2021, in Latin numerals, is MMXXI.