When is too many at a glider meet

Probably already said but Lasham launch over 100 in their annual comp,
every year. It is what they have done for years without too many problems.
If the organisation know what they are doing and the pilots are sensible it
isn't a problem.

As well as 100 or so regularly at the Lasham comps, there are over that number based there all the time, with an organised grid launch on good weekend days (as Nigel P already mentioned).

The home-based gliders can still fly during the competition week, launching before and after the main grid, so sometimes the continuous stream may be 150 to 200.

By coincidence, before this discussion started I watched someone's video on YouTube, starting and ending at Nephi. It didn't look like a small place to me.

People can land sensibly in rather smaller areas without real drama. A retrieve instruction was go along the road to - for about 20 km until you come to what looks like a gliding club. There were about 30 juniors in about 350m by 150m. Or at a pre-Worlds 18 or 20 at a small strip in a quarry at the side of a lake.

One of the biggest mass arrivals I've seen was about 60 gliders in 3 or 4 minutes, but even that only used a fraction of the space available. mostly direct landings rather than circuits. But for those doing circuits the instructions should avoid clashing with direct landings anyway.

I'm not counselling complacency, and people do need to keep their wits about them, but at least avoid wildly exaggerating the risks.

I will add this, even on a big field that is clear, that STILL does not mean someone (that stopped thinking) can't land in a bad spot and run into parked aircraft.
I know, I saw the aftermath at our field (no injuries to people, broken glider, broken towplane).

Max number of gliders has a lot to do with field size & approaches. Huge field with 360* of flat approaches should be able to handle more gliders that a small narrow field with trees on multiple sides.

I have never been to Nephi, so I can't comment on the site.