”Will” 在什麼時候可以用 ”wills”

Hi,

The word" will" in your short sentence carries meanings like

" strong desire" and " volition" and by all means it can be used as a countable
noun.

Also, you are talking about them-- some people there in your sentence,
using " wills" would be necessary, but if you talk about one person ,
you should write " This girl has a very strong will. I am sure she
will go on working for her impossible boss."

One more thing, don't forget to capitalize the first word in your sentence.:)

Regards

Uncountable nouns are materials, concepts, information, etc. which are not individual objects and can not be counted.
The word "will" means 意志,毅力 here. Therefore, it is an uncountable noun.