Repository and Unit of Work asp.net/mvc/tutorials

I'm so confused after reading so many blogs I'm still not sure which way to go and I need help with Repository/UOF pattern using EF 6.

As I was following the example here asp.net/mvc/tutorials and got stuck in "Creating the Unit of Work Class" here, I was wondering: why is that author creating repository Department/Course inside the Unit Of Work? is that best practice? What if I have 20 different entities? Will I be instantiates the repository in the UOF?

public GenericRepository<Department> DepartmentRepository
public GenericRepository<Course> CourseRepository

//sample code extract from the link and you can see here its instantitates the repository in UOF which is the best practice?

public class UnitOfWork : IDisposable
    {
        private SchoolContext context = new SchoolContext();
        private GenericRepository<Department> departmentRepository;
        private GenericRepository<Course> courseRepository;

        public GenericRepository<Department> DepartmentRepository
        {
            get
            {

                if (this.departmentRepository == null)
                {
                    this.departmentRepository = new GenericRepository<Department>(context);
                }
                return departmentRepository;
            }
        }

        public GenericRepository<Course> CourseRepository
        {
            get
            {

                if (this.courseRepository == null)
                {
                    this.courseRepository = new GenericRepository<Course>(context);
                }
                return courseRepository;
            }
        }

        public void Save()
        {
            context.SaveChanges();
        }

        private bool disposed = false;

        protected virtual void Dispose(bool disposing)
        {
            if (!this.disposed)
            {
                if (disposing)
                {
                    context.Dispose();
                }
            }
            this.disposed = true;
        }

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }
    }