To answer your question:
For the number of plants, you calculate between 1 sq ft and 2. In this case 140 cm * 140 cm equals 4.6'* 4.6' = 21 sq ft, so you could place between 10 and 21 plants. That been said, light wise you need at least 50 to 60 watts per sq ft, which means 1000 to 1200 watts of HPS, which means double what you have. I would use a space in that tent of 1m * 1 m and place 4 to 9 plants, that will give you good buds, else if you try to use all the tent with that light, the buds will be as full as you would want.
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