Hi Tylt33,
it depends :)
It depends on the cultivar : Heavy grower or small grower.
Are you growing them / not waiting for fruit just green for cuttings ... Then anything starting with 20 cm is enough. As long as the leaves can get light .
For me , if it is for fruit production, 1,5 meters in between two trees is the minimum ... In natural light. If you go with artificial lighting ...
It depends as well on you climate. Here, the trees don't have time to ripe the fruit from a root sucker - even BT was not able.
Did I mention that 1.5 meter is what I do ...
But of course every tree is kept under 2 meters of height ... To be honest, the winters here help me in that ... a bit too much some years ...