Customising word separators in vi

Is there a way to add underscore _ as well?

:set iskeyword-=_ 

What is, and is not a member character to keywords depends on the language. For help on iskeyword use :help iskeyword.

In case you're using vim, you can change that by setting the iskeyword option (:he iskeyword). If that is not an option, you can always use ct_ instead of counting.