In 1918 Why did a person have to carrie a CANADA Registration Board card? What are the numbers for in the top right hand corner for?

The Canada Registration Board was established in 1918 to compile information about individuals for the purpose of citizenship, pensions, or other important things. All Canadians were required to register (think of it much like today's Social Insurance), and the information compiled prior to 1940 is available only to the registered person. The National Registration of 1940 was the first to be available for search by anyone willing to pay a fee.

The number on the top right would likely be this person's registration number.