How many birthdays are celebrated in McDonalds (in the US)????

This might be helpful: 

"According to Spike, McDonalds hosted more than 17,000 parties in
2002, attended by more than 480,000 people. That's 27 per party, but
(based on my observations) that probably only includes the children,
so we may well be talking about over a million people (or more likely,
a rather smaller number attending multiple parties!)."

http://www.ordinarygweilo.com/2004/02/its_party_time.html

I figured what the heck, I'll contact their corporate office.  Here's
what they had to say, and good luck in your search.

"Thank you for taking the time to contact McDonald's.

As you may know, approximately 85 percent of McDonald's restaurants
are owned and operated by independent businesspeople. As independent
owners, McDonald's franchise owners have the authority to make certain
operating decisions as they relate to their McDonald's restaurant
operations and that would include birthday parties. Because the
franchise owners handle their own birthday parties, we do not have the
information you have requested here at the corporate office.

Again, thank you for contacting McDonald's. We hope to have the
opportunity of serving you again soon under the Golden Arches.

Tina
McDonald's Customer Response Center"