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Hi @coinsandchains, it depends on the size of the jar and on the species you put in there.
The springtails feed themselves on dead plant matter, so if you bottle an evergreen or just moss (as I did) a couple of them should suffice. They're going to reproduce anyway.

I had troubles in picking the right amount of them as they're supertiny and I picked them from my vermicompost bin, so I accidentally collected also a few eggs.

Well, you would think that it would eventually balance it's self out, as long as the initial imbalance was not too great.

Yes, it should.
If not I'm going to remove some of them myself and put them back in the compost bin