The correct form is: Der Stundenansatz des Auftragnehmers.
The stumbling block is the word 'Stundenansatz. It is a German compound noun, probably pertaining to some hourly base charge or wage. "Des (of the) Auftragsnehmers" (order taker) is genitive, singular and grammatically correct in the original sentence. And so the English sentence could be: The base wage of the order taker. But it's a little unclear without context.
Auftragsnehmern is plural. It requires a different sentence structure, such as: Der Stundensatz, der den Auftragsnehmern (plural) zusteht, is $.. (The hourly wage for (of) the order takers is $...).