random Mammillaria's with 2 that are looking to be very variegated?

hi 2deep, I am having a first go at growing cacti from seed and have a small pot that looks like this, with lots of little seedlings sprouting up. what do you do when they get too big for the pot, transfer each one out with roots attached? cheers.

They dont need to be pricked out at this stage yet. Melocactus are slower growing than columnar cacti like Trichocereus(Echinopsis in Europe). When they are larger, ill prick them out into the same pots, but evenly aligned with only 9-15 in each, to make the next upsizing easier. Approximatly half of all my seedlings get grafted. (to pereskopsis atm)