You can plant your beet seed a couple of different ways. You can choose to plant your seed about an inch apart and then when they germinate, thin them so you have a beet every 3-4 inches. You will have a harvest large beets this way.
Or, you can plant beet seed the way I typically do which is to sow a seed every inch or so and then I thin them out every 2 inches. This way, I have two harvests. When the beets have grown to about 1.5 inches I selectively harvest them as baby beets. These, in my opinion are the most delicious and tender. We also eat the greens in salads, on sandwiches or chop and freeze them for smoothies and quiche.
I leave the rest of the beets to continue growing, harvesting them as needed for meals. A larger beet will store longer through the winter and they still taste delicious. When they get about 3 inches or larger, I harvest the rest for winter cold storage. And again, I freeze all the greens.