It also depends how long you wanna veg. More plants, faster veg. Pure sativas, if that's the case, I would not put more than 3 as they can get out of hand at that space quite easily if treated well.
Currently I have DWC bubbler, 2.5x2.5 for veg and 3x3 for blooming, using 2x20L buckets one plant each. Flowering third week. These clones are vegged about 6 weeks to get full area. if there would be one more plant the veg comes down like almost three weeks! Hydro is fast...but I'm also quite experienced in growing and controlling climate, which plays a major role in fast growing.
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