Kalle,
It depends on what you call an S6. If you include the S5, T6, Greyhound, Ayredale, Mastiff, D33, E33, and H33; or in other word all the models that used the YF prefix frame, then about three thousand. Lowest frame number reported in the LDMCC Registry is YF118 (they typically started at #101) and highest is YF3143.
-Doug
[fix typos. 17Sep20 -Doug]