The problem lies in how you're creating the string.
Please excuse my previous answer, I misunderstood your question:
Simply put, the declaration should be as follows:
char str[] = "1,2,3,4,5,6,7,8,9, 10, 12";
Next, you can use strtok to separate the string into an array of strings omittied the separator (which is in your case the comma), then pass the array members to atoi
Now, why is your code not working?
First, characters should be surrounded by the apostrophes or else the compiler will take the number you pass literally as the ASCII value.
Second, arrays in C like this:
char str[] = {'1', '2', '3', '4', '5'};
don't mean a comma separated string, these commas separate the ARRAY members, each in its own index and not as a whole string.