Functional way to generate file names in Scala

Ok, I want to generate temp file names. So, I created a class with var tempFileName and fileNo such that it creates files like

BSirCN_0.txt
BSirCN_1.txt
BSirCN_2.txt

But, to do this I have to keep count and the way I am going it is calling next() function of the class which returns the filename in sequence (should return BSirCN_4 in the above case. Now this goes against FP as I am modifying the state i.e. the count of names in the Object. How do I do it in a functional way. One way I can think of is keeping count where the function is called and just concatenate. Any other ways?