It can be countable or uncountable. Most of the time, it will be countable. If you are talking about "souls" as individual units (each person has a soul), then the word is countable (if there are twelve people, there are twelve souls). If you are talking about "soul" as a quality or a type of substance, then it is uncountable. "This music has a lot of soul." The uncountable form is fairly rare. Particularly in religious contexts, it is very common to talk about human "souls," plural. They're definitely countable.

Souls as dead people is countable. Many souls were lost that day. "soul" as something that a song has is uncountable. That song sure has lots of soul.