Definitely place only 1 seed per sponge. When they sprout, they need all the space they can get, so multiple seeds would just each get less space to grow, leading to poor plant growth.
The key is to start with a full batch of sponges(max minus 1 hole for the air pumps/etc), so that when some of them inevitably fail, you still have a few left that are going strong
My current grow(my first), i started with 6 seeds, and 3 didn't sprout. If I had started with only 3 at first, I might have nothing to show for that first week or so!
...but yeah, 1 seed per sponge