afni.nimh.nih.gov

Sadly, clustsim cannot be used with less than 14 subjects (or rather, 14 sets of data). Per the help on down the page:

 ++ If you have less than 14 datasets total (setA & setB combined),
                 this option will not work! (There aren't enough random subsets.)
               ** And it will not work with '-singletonA'.

There are also some details in there about the min per group I believe - if that is important to you as well.