How to count how many word in string?

I want to know how to count how many words are in a string. I use strstr to compare and it works but only works for one time

like this

char buff = "This is a real-life, or this is just fantasy";
char op = "is";

if (strstr(buff,op)){
    count++;
}
printf("%d",count);

and the output is 1 but there are two "is" in the sentence, please tell me.