I did 8 mainline on my critical kush and had eight 12 inch colas. As far as space I have plenty but having said that my indoor will most probably be 20 of these clones mainlined to 4 or 8 like you say. There is no popcorn on these they will fill in. Our heat is very dry , late in the afternoon they tend to feel it a bit. I have just written to female seeds to try to get a definitive answer on harvesting time. So, we'll see what they say.