Among professors with the highest popularity, how many of their students have research capability of 5?
highest popularity refers to MAX(popularity); research capability refers to capability; capability = 5;
SELECT COUNT(T1.student_id) FROM RA AS T1 INNER JOIN prof AS T2 ON T1.prof_id = T2.prof_id WHERE T1.capability = 5 ORDER BY T2.popularity DESC LIMIT 1