when do i break up seedlings

This my first year useing the jiffy 72 ct. with pellets starter house all of my pellet have have reach the top of the dome and many have 5 or more in them. because of my nails i could not only put 3 seed per unit. what i was wondering is when do i separate the plants

When they have their 2nd set of leaves you can separate them

You should remove the cover once the seeds have sprouted.

Allow some air to circulate around the plants once they show green sprouts, by this I mean remove the cover, if cold, replace early evening till they become accustom to the change in temp when cover is off. after a few days it should be OK to leave the cover off.

Dont allow the seedlings to dry out but DON'T flood them with water either, as the seedlings get growing you will notice they grow taller and develop some proper leaves, this is what flowerjen was referring to (2nd leaves or true leaves) called true leaves because they are tiny leaves that are the ones that are the same as the type of the leaves we recognise on the adult plants.

To separate the seedlings, either knock the seedlings out of there container along with all the soil and use a blunt pencil or other tool (some people use a plant label ect) use this to very gently separate the seedling, Hold the little seedling by the leaf and use the tool to very gingerly pull the seedling free, keep hold and place it in it's next container either in a tray of compost OR into it's own SMALL pot to grow on.
At this stage our seeds are very easy to go into shock, disease or even insect damage so keep an eye on them and and check for signs of the little plants looking sad, falling over or whatever, don' over react, check to see if they need water ( Poke finger into soil, if wet, no water needed if dry give a small amount) wait a few hours to see IF that was the problem, or check to make sure plants are not too close to hot sun like on window sill, if yes (wilted plants look sad) either place a folded sheet of newspaper against the window just to offer some shade OR take the plants away from window to where there is good light but not hot sun. af for checking insects, nibbled leaves are a good indication, or lift leaf up and check for greenfly or other SAP sucking bugs, if there is, please dont go out buy spray's, just mist the plants with tepid water and do underside of leaves too, or be brave and kill the tiny little creatures between thumb and finger,
After you have been working with your tender seedlings, always wash your hand as you dont want to spread diseased from one to another.

Make sure you harden the growing seedling off by placing them out doors in day time and bring inside evenings for a couple of weeks before you leave them out in garden where you want them to grow and develop. by this time you can offer a feed and always deadhead the flowers to keep the plants producing more flowers if it's for flowering plants for veg, you don't remove flowers at all unless like pumpins or that ilk who need to put energy into fruit rather than hundreds of flowers.

Hope all this helps you understand about growing from seed, BUT, you will find your own way as you gain experience and in time you will be growing more seeds than you knew to be possible ha, ha, ha.
Take good care and have a great growing season.
WeeNel.