十 is a single vertical stroke crossed by a horizontal one — a complete cross. Ten was the end of counting on a knotted cord, the point where a tally was 'complete'. The crossing lines came to mean both 'ten' and 'whole, complete'.
十 means ten and, by extension, completeness and perfection: 十全十美 ('ten out of ten, perfect in every way'), 十分 ('ten parts' = completely, very). Numbers build on it cleanly: 十一 (11), 二十 (20), 十个 (ten things). Chinese counting is gloriously logical from here.
A vertical and horizontal crossing — a complete cross, TEN.
Chinese numbers are beautifully regular: 十一 = 11 ('ten-one'), 二十 = 20 ('two-ten'), 二十一 = 21 ('two-ten-one'). Learn 一 through 十 and you can count to 99.