A 1000W HPS would be sufficient enough for nine females. Especially since you're growing in a pretty confined space. Be sure that you are reflecting as much light as possible, either via Mylar or flat white paint. You can probably expect at least 1oz. or more per plant from Northern Lights with decent ventilation, a good nutrient regimen, and plenty of reflected light.

The biggest issue you might have to deal with will be limitations on space. NL is mostly Indica, and will likely grow bushy, as opposed to vertically. You can learn more about thinning & training your plants to get the most out of the vertical space available.

A 1000W fixture will generally supply enough light to cover a 6.5 foot by 6.5 foot grow area. (30 plants or less) But, as with everything, results will vary depending on your own circumstances.