When the seeds first sprout, the first leaves that show are the seed leaves or cotyledons, that are shaped differently than any of the rest of the leaves will be. After that, the next pair and all of them after that are true leaves.
https://1.bp.blogspot.com/_6KRo-dmHHWA/S ... dlings.jpg
green bean seedling with cotyledons and true leaves.
So I definitely wouldn't try thinning them until they have true leaves. There is no time when the roots would make it too hard to pull them. You can always moisten/loosen the potting soil and lift the plant out from underneath. What you are worried about is when they are competing with each other too much for light, water, nutrients. So if the leaves are touching, if they look crowded, if there's not air circulation space between them, if they stop growing very fast would be signs that they really need to be thinned.
I like to let them grow for a little while, maybe until they get the second pair of true leaves, because then it will start to be clearer which ones to cull - you want to cull the weakest ones, the ones that aren't growing as fast or as well.
evtub asked about the container size, because bean plants grow mostly vertically and don't take up a lot of room. Unless your container is quite small, there may well be room for more than one plant. One bean plant may produce 20 pods or so.