Hello! This is my first post here :)

I recently began growing herbs in my backyard. They are in tiny plastic starter planters with some potting soil, about 6 inches in diameter.

I planted four seeds to make sure at least one sprouted, but the entire area of the planter is covered in little thyme spouts now. I was told to try and only let one grow per planter, but it looks like they are all thriving (hooray).

Is this healthy for them? Will they choke each other? The plan was to move em to a bigger planter once they get a few inches high, but they've slowed down in growth. I am worried about my lil' tiny seedlings.

Need thinned yes, where are you located? Thyme likes cool weather, but after sticking their heads above ground they will stall so the shallow root systems can grow. My thyme is already dying back since we have hit 90* here, even in shade it is hard on them. You might try using a spoon and simply scooping them into other containers, but most seeds do not like being messed with, so it is easier to just thin and reseed other spots

Welcome @dunamisdeos

I would not worry about thinning thyme seedlings. When I sow thyme seeds for growing, I liberally sprinkle seeds over the cell pack or pot and just let them be. You will end up with a nice, thick stand of thyme when you are ready to transplant or plant out. I grow 100s of plants each year for our MG plant sale and this method has not failed me for thyme. It makes for a nice full plant. A 6" pots sounds OK for where your plants are at this stage.

Once your thyme has a few sets of true leaves, you can transplant it into a larger container if you wish. Don't know where you're located, so not sure about your plant out date. You may wish to edit your profile to give us an idea of your growing zone so we can provide more specific advice.