Group buys are not typical so there really isn't an answer.
This is simplified but should give you some idea:
When you get a quote to CNC the case it will cost X to setup the machine (while probably scrapping the first effort) plus Y per board where X>Y. If X+10Y/10 is cheap enough that 10 people buy in the GB would go ahead, sometimes you need 25 buyers to make it worth doing. Or 50. This is why many buys (used to? the hobby has grown so much maybe it's no longer a worry...) have an MOQ (Minimum Order Quantity)
If you have ran a GB before and the board was great people are more likely to trust you than if you're a newbie who could be well intentioned but uses a sloppy factory and delivers a poor product, or worse string people along for six months until paypal protection ends then run off with the money.
The final factor will be what you're trying to sell - a niche ortho board will not shift anywhere near as many as whatever the flavour of the day is.
When you get popular the other end applies - you can only handle so much quality checking and packing and 50 keyboards on a pallet is going to take up a good amount of space in your garage, so some buys are capped for legitimate reasons. Others are supposed to be 'exclusive' so the opportunity to buy 1 of 10 is either raffled or done first come first served at a pre advertised date and time.
I'm sure LZ and RAMA could sell hundreds of boards, whether they choose to is another question.
