Mong - it's all relative to the size of your plants. When i first got my 250, i put 4 plants in, and flowered them pretty small, at 10" or, and they got to 20-24" by the end. BUt i still had space and threw a couple of small clones in 500ml pots in the spaces to soak up any extra light and give me a little more smoke. I currently have 2 larger plants, at 18", which I flipped today, and I hope won't go much over 24", or i'm dead out of space (must take some pics tomorrow, been too long). I have contemplated doing 9 plants in 2 litre pots , about 18" finishing height, with one main cola each and some nuggets...sure it would work well but never tried it. It's a case of suck it and see I guess. Verde has a good point, some form of training is key with the 250, as the light doesn't penetrate too far.
One thing i will say though, more plants equal more plant matter and soil, so moisture is increased and you need to have better circulation and extraction
