11 is the one beyond 10. K is the 11th letter in a number of alphabets - for example the English, Chaldean, Greek and Latin alphabets. It’s the last letter of Magick. It’s attributed to the god Jupiter, whose vehicle is the eagle, symbolic of Magick power in its feminine aspect. K is also the Khn, Khou or Queue symbolised by the vagina.
Because the number 10 was/is regarded by Qabalists as the stable number of their system of divine emanations (sephiroth) the number 11 was considered accursed, because it was/is outside their system. It represents the Magician outside of and therefore separate to the ten sephiroth of the Tree of Life.
It is an incredibly important number within the Thelemic system - and the Thelemic English Qabala.
Hope that helps.