Thinning depends on the soil, season and the amount sown.

If the weather is warm, carrot seeds should germinate in about 3 weeks and will need another 3-4 weeks of growth before thinning.
If germination isn’t too thick, they may not need to be thinned as the removal of the spring onions (which we sowed in with the carrot seed) will create gaps.
Also, if you prefer baby carrots, there may be no need to thin.