I received a package of 50 mint seeds. They seemed small so I just planted all of them in a wide 10 liter pot.
Was that too many seeds in too small of an area? How much is too much?
Well, seems you'll have a lot of thinning and pricking out to do if they all germinate. If you imagine each seed as a single mint plant, which needs 18-24 inches of space around it as it grows on, if all 50 of your seeds germinate, you will need to transplant each one into individual pots, so in theory, that's 50 pots. If you sow too thickly, or too many in too small a space, it's quite difficult to separate out individual seedlings without damage; if they're left too long in a crowded state, you risk fungal infections because of lack of air flow round them, but if you try to prick them out too early, the roots will be too fragile to cope.
Just wait till they germinate - then you'll need to remove some if they come up as a thick mat; just pull out any you don't want, but make sure the soil in the pot is damp first, and ease them out with as little disturbance as possible. Once those left get their second set of leaves (true leaves, not the cotyledons, the first set) then you'll need to prick them out, as many as you need, into individual pots, then pot on into larger ones until they're large enough to either put outside or in a much larger container.
If you wanted, say, 2 mint plants, then sowing 20 seeds on a seed tray or pot would be slight overkill, but after germination, you'd just select say 5 of the best, biggest, healthiest looking ones for potting on and bin the rest, then bin what you don't want of the 5 later on, once they're growing properly and you can see which are the bushiest, liveliest plants.
I think you'll be fine as long as you transplant the resulting plants into clumps of say, three to five seedlings per pot. By tranplanting seedlings as clumps instead of individuals you will keep root disruption to a minimum. The multi-seedling transplant technique works fine for plants that normally grow in clumps - as many herbs do. (This, btw, is how commercial nurseries create those nice, full-looking pots of herbs to sell.) Using this technique would not be a good way to raise other things like tomatoes, however.
Btw, I just reread your post - if you scattered them all across the top of the soil in a 10 litre pot, then you will not need to transplant for a long time, maybe never. All the seeds that sprout (all 50 probably won't grow...) should have plenty of room to grow, and all that will happen is your pot of mint will become fuller sooner than it would have otherwise. Once it fills the pot, you may want to divide it and give some away, or if you are a real mint lover, start some new pots!