A descrição do sql dessa aula está com um erro, faltando o count(*) as total na hora de passar o sql pro Program.cs
ficando assim o SQl:
var sql = @"select a.* from actor a inner join (select top 5 a.actor_id, count(*) as total from actor a inner join film_actor fa on fa.actor_id = a.actor_id group by a.actor_id order by total desc) filmes on filmes.actor_id = a.actor_id";