hi deri. i too am on my first grow and couldn't agree more about how fun it has been learning about everything. i plan on using a combination of LED and MH/HPS. i have done a s*#t ton of research on lighting and when i was planning placement of my LEDs i found a really helpful diagram that basically shows you 3dimensionally the best way to place your lights. of course i can't find where i saw it right now...

...but i'm gonna describe it and maybe that can point you in the right direction. it was basically like if you placed say a sheet over your light and it stayed straight at the corners. so that means that the further up your light is the larger its footprint so to speak. if you have it very close then maybe your footprint is only a few inches all the way around than the actual size of your light. but if you've got it 40" off the ground then your footprint would be much larger, probably 14-16 inches all the way around larger than your light. i hope that makes sense.
so i think that placing them parallel longways would be a "waste" of light, unless you're growing in a long narrow space. if what i've read is correct, placing them far enough apart so that each of their footprints are just touching is the recommended way to use them to maximize coverage and efficiency. so it looks like from your pic what fozzy said is the best way and then use your grow space footprint and work backwards from that to determine the best way of arranging your lights. that will also help you answer the how many plants question. i wish i could find that dang diagram!!
also, i found this from Nebula's tutorial very helpful:
"LED grow lights tend to need a lot of space between the lamp and your plants, which means you need a tall grow space to get the best results. This is actually the main thing holding me back from trying LEDs more. Smaller LED panels should be kept 18" or more away during the second half of the flowering stage to avoid
light burning your buds (buds can be burned from too much light even if the temperature is cool), and some of the bigger models need to be kept 30" or more away from the buds. When in doubt, contact the manufacturer!"
that came from the LED section here
http://www.growweedeasy.com/cannabis-grow-lights
i did some testing while i was waiting for my seeds to arrive where i just ran the light in the space and took temp and humidity measurements to get a baseline for when i did get to actually growing. i CANNOT stress enough how right Nebula is about the cooler temperature of LEDs being very misleading. that's unfortunate because the goto rule of thumb that i've read everywhere is if it's too hot on the back of your hand for 30 sec then it's too hot for your plant. that is not gonna work as well with LEDs because they run so cool.
anyway, hope that helps.