it's kinda up to you...if they are in a polytunnel and you have no wind and plenty of room for them to grow, I would give them 36" spacing...the larger varieties will still grow into one another...a good healthy trinidad variety will get up to 5 feet in diameter and 6 feet tall...so your spacing depends on the mature size of the variety you are growing...for instance white habanero's can he spaced at 18" because they are smaller plants...