Okay, what I'm about to describe below is not the correct way of transplanting, in fact it's very risky and I would never do it with lithops and other mesembs because they rot easily. Yesterday however I was facing a challenge of transplanting over 60 Anacampseros seedlings (An. retusa fa. rubra, An44) from the 5cm container they were growing in. Basically it looked like this:
My goal was to give them more room to grow but at the same time not to use more than 4 new 5cm pots. Also, I wanted to do it quickly. Anacampseros are easy and don't rot when young (rotting sometimes happens to adult plants with thick roots) and I have way too many of them to worry.
The method is simple: fill up the new container with dry pumice, then water the top layer (really just as much as necessary, don't make the whole thing wet), poke holes into it and stick in the plants.
This way you can fit up to 25 plants all neatly into one 5cm square pot quickly and with little effort.
Remember, if you're not afraid to lose plants and need to do the transplanting in a hurry this is the method to go for. BUT if you use it for mesembs the risk is very high - the seedlings might turn to mush the next day. I warned you! In case of Anacampseros, I have transplanted all my seedlings (hundreds of them) using this method a couple of months ago and had zero losses (you've probably seen the LQ pictures of my huge plantation over on Twitter). Young Anacampseros are low-maintenance.