I entered 20 into the field and chain lightning hit 20 targets, so it worked for me. See if you can make chain lightning hit more than 16 targets in a test map. If you can, then there would be something else in your map causing the problem, and if you can't then the problem would be with getting it to hit more than 16 targets.
Max amount of targets is 21 no matter even with shift for if it is past 21 then the hero just acts like its paused and its control panel goes away and wc3 blows up.
Yet again, if you want more then 21 targets, you can always trigger a chain lightning spell from scratch..
Not really that hard... If ya need directions ,tell me.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.