It will help to find what variety you have, as some varieties need much wider spacing than others.
Here is an idea of what you are looking for via spacing:
- 2-3' tall matured: Space 10-14" apart
- 3-4' tall matured: Space 12-18" apart
- 4-6' tall matured: Space 14-24" apart
- 6-8' tall matured: Space 16-30" apart
- 8-12' tall matured: Space 20-36" apart
- 12'-up: Space at least 30" apart
The wider spacings are better for branching varieties. Sunflowers can be grown tighter, but the result may be unnaturally small heads, on top of very long stems. The wider you space, the shorter the stems you will get, and also larger heads.
You can thin them anytime at this point, but I often wait until the plants are 4-6 inches tall, so that I can be more sure of the vigor levels of the young plants. When you are thinning the seedlings that are very close, place a finger on each side of the seedling you wish to keep, to hold the soil in place while you pull the surrounding plants with your other hand. This may not be necessary, but safe is always better than sorry.
The burgundy color on a sunflower stalk, if it is surrounded by normal green sunflower plants, usually indicates a reddish tinged to full dark purple flower. However, some yellow flowered specimens have a burgundy stalk. If you didn't plant a blend, and are looking for uniform color, you may want to cull these. If this is from a blend, try to leave as many different growth types as you can. This is easier when the plants are a little older.