Answer: The seed starter mixes are usually formulated with a large proportion of vermiculite to assure good water holding along with good drainage capability. This helps you avoid overwatering your seeds which would cause them to rot. The vermiculite is also inert so it helps reduce the chance of disease such as damping off.
Depending on your level of skill, you could probably use the soil-less potting mix, but I would still top that with a very thin layer of vermiculite as a disease preventive. If you are new to seed starting, I would recommend a seed starter mix because it is easier to use.
Whatever soil mix you use should be premoistened before filling the cells. Use a clean bucket or plastic bag and add warm water to the soil mix, begin with a ratio of four or five parts soil mix to one part water and mix thoroughly. It should be damp like a wrung out sponge. Stir or remix often as the water tends to drain to the bottom at first. It is good to moisten it and let it meld overnight before planting, remix again and then fill the cells.
After planting the seeds, water once lightly to settle the soil and assure good soil to seed contact. A little sprayer bottle is perfect for doing this step.
You can put as many seeds in a cell as you want, but ultimately you will only want one plant per cell, so using more is wasteful. The reason to use two or three is for insurance, to make sure you have one good plant per cell. Once they germinate, you quickily remove all but the strongest best seedling. I hope this helps!
Q&A Library Searching Tips