In most cities around the world you can retake the seminar as many times as you like.
Some organizations may give priority to students who first took the class from them.
In the US, any graduate can repeat in Learning Strategies Corporation organized classes as often as he wants--providing there is room in the class. We always save room for at least 5 graduates, depending on the size of the facility.
As far as being an instructor... It is a time consuming and expensive proposition. I will ask Lynette Ayres to respond in more detail next week. She oversees our instructor training program.