Nihon Ki-in Professional Examination
If I understood right, two professionals are chosen every year by pro exams by the Nihon Ki-in. The Kansai branch chooses one new professional, as does the Nagoya branch, and the women's professional exam determines one more new professional.
The Nihon Ki-in holds two professional exams a year, one in the summer and one in the autumn. From the summer exam, one person passes the exam, and from the autumn exam, two people pass.
Blog from Antti Törmänen, who studied as an Insei in Japan in 2011-2012 and again starting in 2014.