Try this:
"I would like to thank you for your interest, and for the offer to attend __________'s clinical psychology program. I have forwarded your letter to the offer acceptance committee for its consideration, and you will be notified of its decision shortly. Your offer is highly qualified for acceptance, but please be aware that due to the high number of applications that I submitted this year and my inability to attend multiple programs, I cannot accept all of the offers that are extended. Best of luck: I am sure that you will find a doctoral candidate this year."