When you are comfortable that the seedlings are doing well, then feel free to cut one that you don't want, or just pull it out carefully. Throw the remnants into compost bin.

You can also eat it when it is still smallish, so you could pull and eat, leaving behind some to get larger.

I actually plant mine much thicker than that. I divide my mesclun bed in half or thirds using a scrap timber/stick of wood as a visual divider, loosen the soil a bit using my fingers as a rake. Densely sprinkle seeds on top, pat down, then add a dusting of peat. Water well, then add some sticks resting on the bed edges to give it a bit of light shade to get going/germinating. Start the second half of the bed two weeks later. (i keep the un-planted half covered and weed free)
Planting thick keeps it nice and clean suppressing weed growth.
I would suggest sprinkling 'more' seed, if you have it, on the left half of your bed and top with a bit of dry soil if you don't have peat. Water well, and use this as an experiment.

My mesclun bed is a bit different...2.5 x 6ft. but raised as yours is. Divided. Last weekend put in a variety that does not mind the cold. Another will go in that does not mind the heat, (late june)...I get salad all summer as one needs pulling as one is harvested.
Growing thick and cutting with scissors the first harvest, you will get a second growth harvest from your first planting. And no weeds.
...Then back to the cool season mix the end of august for a good fall crop.
I buy three mixes in bulk so i have plenty seed and a better price in bulk.
Any leftover seed i grow indoors in those clam-shell grocery containers for micro greens all winter.
I end up with so much in an area just a bit bigger than yours that i drop off bags to the senior center a few times every summer. (with all that extra zucchini, : )

-not bulk as in enormous...just 1/4 lb amounts.
for example, the braising mix does well early in cool soil...the all-star is great in summer...then braising in the fall...the spicy micro mix i put in weeks ago and it survived a late snow fall without any cover.

-i do grow a variety of head forming lettuce in another bed, but mesclun is always ready to add to a salad of other bigger leaf varieties.

Here is a link that might be useful: salad mix