14. From the 20 numbers 11, 12, 13, 14, ... 30, at least ( ) numbers must be taken to ensure that there are two numbers among the taken numbers whose sum is a multiple of ten.
(18, 22) (19, 21) (20, 30), there are 10 drawers. By the drawer principle, selecting 11 numbers, there must be 2 numbers falling into the same drawer, and the sum of the 2 numbers in the same drawer is a multiple of ten. If taking 10 numbers, one can take $11, 12, 13, 14, 15, 20, 21, 22, 23, 24$, in these ten numbers, the sum of any two numbers is not a multiple of ten. Therefore, at least 11 numbers need to be taken.