One bulb or two? Does it include a reflector like this? What plants are you growing? Just spinach?

Two tubes work better than one because they are roughtly the width of a seedtray, or half tray anyway. Then you position the tubes a couple of inches above the top of the plants. The 24W (actually 22" long) T5 produce quite an intense light because they are small tubes, so you might have to have them 4"-6" above the plants. You will only be able to grow two or three half trays under 24" tubes, any part of the trays more than about 6" from the tubes won't grow well. You can try with one tube if that's what you have, but watch out for seedlings that lean or stretch towards the light. Buy a second tube of this happens. That should answer 1) and 2).

3) I don't like plugs. I just use cellpacks or half trays filled with seed potting mix.

4) Sorry, I don't know about growing spinach. It is common with other seedlings to harden them off in a coldframe when the nights aren't too cold.