How many Christians go to church at Christmas?

Depends on the denomination. Some churches run multiple services on a Sunday; others will have only one.

Most churches will have a Sunday morning service, starting between 10-11am. Some churches will have an early service at breakfast time (8am), and others will have a second service in the evening - frequently with a more study-oriented structure. Most Christians would attend one service as a matter of course, with a second for a specific purpose when required.