Since no one else has responded I'll take a shot. First, I haven't grown torpedo onions, but since that defines the shape I'll assume that the rest of the plant grows like other onions. I plant my "round" onions 5" apart, not just for bulb development but also to provide adequate sun for each plant, so it likely wouldn't hurt for you to plant yours at the same spacing.

Secondly, I guess there are multiple onions in each cell, so you should split them up and plant them individually, otherwise you probably would have a wad of smaller, crowded bulbs at harvest. Multiple onions planted in the same hole would just cause them to compete with each other.

And, if they are like other onions, when they are ready to harvest the tops will fall over - my regular onions always finish up half or more in the ground and half or less out of the ground.

hortster

I grow several hundred Red torpedoes every year and love them. Many are over a lb. Red torpedoes get quite large but grow more vertically. They definitely need separated with 5-6 inches between them. As noted, they grow upward, so there won't be much below ground. Tops do fall over, but it's not as obvious as w/round onions. They don't keep real long.

I grow the OP version 'Long Red of Tropea' which I consider to be the Brandywine of onions. I give them a little more space than other bulb onions, 6 inches, because otherwise they won't get big. The tops do fall eventually, but because these are fresh eating onions, you don't have to wait for full maturity. Even when perfectly cured, mine break dormancy in December, so we eat most of them fresh.