You would either pre-define the number of a given type that youwant in a string and then choose the voltage accordingly, or else you would pre-define the voltage and then decide how many LEDs of a given type you could put in a string.Wouldn't I need to base the voltage anyway on the type of LED's I shall be using?
I think this is the PNP emitter-follower arrangement that dougy83 had in mind:
It has the advantage that the LEDs reduce the voltage which the '47 output transistor is subjected to.