XI or xi is the Roman Numeral for Eleven (11).
The Roman Numeral before XI is: X. The Roman Numeral after XI is: XII.
Note that the 14th letter of the Greek Alphabet (Upper Case "Ξ" and Lower Case "ξ") is called Xi in the dialect of English spoken in Canada and the USA. (Although it is called Si in the dialect of English spoken in England and Scotland.)