Posted: 23/03/2014 at 18:42
When they've grown big enough to handle and have their first real leaves, not just the seedling ones, you pot them on into fresh compost in little pots, or saved yoghurt pots with holes for drainage in the bottom. If they are only a week old they are probably still too young. When you pot them on you hold them very gently by the leaves, not touching the stems, and using a pencil or a plastic plant label ease the seedling carefully out and drop it into a hole in the compost in the new pot having made the hole with a pencil. Push the compost carefully around the seedling and water it. Then when there is no more risk of frost you can plant them in the garden.
As for how many to sow (you sow a seed and plant a plant), it depends how many there are in the packet. A dahlia seed is a reasonable size, you can sow each one individually into modules or about an inch apart in trays. Some seeds, like petunias and lobelia are really tiny, much harder to sow.