Cactus / Ariocarpus seedlings

I repot 25 of one year old seedlings into a 10 cm square pot in spring.
This repotting accelerates the growing speed but gives some casualties of seedlings.
The photo is Aiocarpus retusus v furfuraceus cv Sugri cauliflower repotted last spring.
Fortunately there are no casualties in this pot.
I will do the second repotting in September.