Write SQL query to solve given problem: Among professors with the highest popularity, how many of their students have research capability of 5?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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