About how many people on earth are christadelphian?

No formal number exist, I believe there is an effort to get a number. The number of living Baptized Christadelphians could range from 50,000 to 100,000. These numbers don't include children, those learning the gospel, or smaller countries.

This link could give you a visual on where Christadelphians are located worldwide.

http://thisisyourbible.com/index.asp?id=37