So I am using the list folder task as a way of counting the files in a the folder, using the task variable result "Number of files listed". I then want to perform a calculation on that but it appears that the variable type is string. Why? Can it be converted?

Sounds like you're finding out like I did that you can't just do a calculation like {JOB(Active,Variable,Days)}+2 as the variable Days in my example is a string (with a value of 3 say) and you will end up with a string with a value of "3+2".

You need to use a math function like this:
{MATH(Add|Integer|{JOB(Active,Variable,Days)}|2|#0)}
which will result in a result of 5 as expected. The math function appears to do the conversion for you. So, have a look at the "Math" section in the Variables dialog.