mkdir sa{1..50} # 50 directories from sa1 through sa50
mkdir -p sa{1..50}/sax{1..50} # same but each of the directories will hold 50 times sax1 through sax50 (-p will create parent directories if they do not exist.
mkdir {a-z}12345 # 26 directories from a12345 through z12345
mkdir {1,2,3} # comma separated list makes dirs 1, 2 and 3.
mkdir test{01..10} # 10 directories from test01 through test10.
mkdir -p `date '+%y%m%d'`/{1,2,3} # same as 4 but with the current date as a directory and 1,2,3 in it.
mkdir -p $USER/{1,2,3} # same as 4 but with the current user as a directory and 1,2,3 in it.