concentration you used of Auric acid for preparation is the concentration of AuNPs. only you must mention is the total volume of AuNPs you prepared and for that auric acid and other stabilizing or reducing agent you used.
other then that you need to mention the volume you are using of that AuNPs for particular experiment.
there is no particular concentration in milli molar or PPM unit for AuNPs.
but yes, you can calculate auric acid in moalr concentration for the same.