I'm just going to talk from personal experience.
Firstly, you will have to cut the plants.
My last one almost grew to 2m, and you need a lamp about 3 feet (1m) away from the plants or they will burn.
Theoretically its possible to use only one lamp for this, but I'd personally use 2 if finances allowed for it, at 2m intervals.
as for how many plants?
depends on what your aiming for, if your aiming for one big grow at a time, quite a lot of plants, to be honest, I'd say about, 25 safely.
So I'd start with 50, judging that by the law of averages, 25 will be male and 25 will be female,
Judging as you stated your measurements in metric I'd hazard a guess your english, although I could be wrong.
You havent spoken about any windows or anything, so fresh air and circulation would be a big issue.