late Old English scoru "twenty," from Old Norse skor "mark, notch, incision; a rift in rock," also, in Icelandic, "twenty," from Proto-Germanic *skur-, from PIE root *sker- (1) "to cut."
The connecting notion probably is counting large numbers (of sheep, etc.) with a notch in a stick for each 20. That way of counting, called vigesimalism, also exists in French: In Old French, "twenty" (vint) or a multiple of it could be used as a base, as in vint et doze ("32"), dous vinz et diz ("50"). Vigesimalism was or is a feature of Welsh, Irish, Gaelic and Breton (as well as non-IE Basque), and it is speculated that the English and the French picked it up from the Celts. Compare tally (n.).
From Latin vīcēsimus (“twentieth”) (from *vīcēnssos, from Proto-Indo-European *wi(h₁)ḱm̥ttós, with the -imus ending of decimus (“tenth”)) + -al (“suffix forming an adjective”). Compare Latin vīgintī (“twenty”)