Seeds planted too close, now what?

Depends on the plant, with salad leaf crops like Rocket I keep them in the same pot they germinate in, then transplant straight into the soil when they are big enough (and "hardened off").You'll easily get a dozen or so plants in a pot your size.

"Hardening off" is transitioning the seedlings from the nice warm indoors to outdoors, you need to move them outdoors each day and bring them in at night so they gradually get used to the cooler outdoor temps. Do this over a week to 10 days.

For other plants like tomatoes for example they need to go into individual pots as they don't like being crowded.