How many solution paths are there inside the 2nd most popular repository?
more watchers mean that this repository is more popular; most popular repository refers to max(Watchers);
SELECT COUNT(DISTINCT T2.Path) FROM Repo AS T1 INNER JOIN Solution AS T2 ON T1.Id = T2.RepoId WHERE T1.Watchers = ( SELECT Watchers FROM Repo ORDER BY Watchers DESC LIMIT 1, 1 )