dd if=/dev/urandom count=200 bs=1 2>/dev/null|tr "\n" " "|sed 's/[^a-zA-Z0-9]//g'|cut -c-16
How do I repeat it set number of times?
it will repeat
If the point is to learn shell programming, see cYrus's answer.
If the point is to generate a password, many distributions ship with pwgen.