I don't have a lot of
experience with flowers, particularly, but I've been growing herbs and veggies from seed under
lights for a while now. I personally start with 5 or 6 seeds per cell, or more if the seeds are tiny and I can't be bothered to separate them more carefully. I start early (beginning/mid February - here in central Maine I won't be planting these into the garden until the end of May at the earliest), and when the plants have a full set of true leaves (the second set to arrive, the first set won't look like the actual leaves of the plant you are growing, but the second set will) I separate them into their own cells for the first pot-up. I will pot them up again before they go out into the garden. It's a bit more work, but I like the extra head start all my plants get from this attention.
Once you get as obsessed as me, the biggest problem is finding
enough space for all your trays of seedlings.
Here's a couple of pics of my seedlings at the moment: