Now, the first thinning should be done when the carrots are about 2-3 inches tall. At this point, you only have to thin them to about an inch apart. I usually just look for the one that looks the best and is the tallest and then pull the others around it out. Once they start getting bigger, you can do the final thinning to about 2-3 inches apart. The tool of choice for carrot thinning is tweezers. It's hard to get your fingers in there without disturbing the carrots that you are leaving in.
There are a lot more carrots than it looks like in this picture. Several are right on top of each other, so they definitely need to be thinned, even when they are this small.