It can really depend on the cuke- some are seed heavy, some not so much.
But something to note is, don't pick your cuke at fresh eating stage for seed saving. The seeds are usually to immature at that point. You want to let your cuke get overgrown, bloated, leathery/hard skinned... pretty much just grossly unedible overgrown. That cuke will have mature seeds in it.

Thx for the help. But I'm tired of the word "depends". It says nothing really....

Ok... so a cucumber can have hundreds of seeds.

If it was well or multiply pollenated, it will have more seeds than if it was not. How many male flowers were open at the time, how many insects picked up pollen from those flowers and pollenated the female flower, how long that flower was open to receive pollen...

Watering, nutrients, temps, and harvesting time can all also play a factor. Which particular variety of long Asian cuke is important too- different varieties will have different amounts of seed.

But a cucumber picked at the fresh eating stage will be unlikely to have viable mature seed, regardless of how many seeds there are.

I saved 3 such cukes for seeds. They are getting long and fat still, but still relatively green. They plants receive full sun, regular water and we have a lot of bees and ants. So it seems I can get hundreds of seeds. I think this is enough for me now.

I also save a single Kirby type of cuke. It is not something I like in particular, but just want to save some seeds for future or for sharing....

When you pick your green cuke to eat it, take it inside and slice it open...do you see a lot of seeds? They'll be slim, small, fleshy. Lots of water. Looking at that cut up cuke, can you estimate the number of mature seeds you will get if you let another of the same kind of cuke mature, bloat, leather-up as it grows older and wiser?