The figure of 30 to 50 (or similar order of magnitude) is my impression after attending Kau Yan's parents briefings (both internal and external). Yes, my kid goes to KY too.
Since the school is a private school, the Vice Principal has explained that the school does not have the needed resources to deal with students with learning difficulties. He reminds parents to send such kids to aided or gov't schools instead b'cos there will be extra subsidies for admitting these students.
I understand that all not Kau Yan K.3 students can go to its P.1 class. The school will allocate about 30-50 places for its K.3 students, meaning that they would need to compete (based on conduct and academic performance).