Plant count won't be the issue, even if they are all female and you can keep them all. Lighting will be your limiting factor. I know of a recreational grow in Oregon that keeps their babies in one gallon containers that are only like 3 inches apart. The plants each get almost 3 feet tall and they have to water 3 times a day but they grow some amazing flower. The second those leaves start to touch, flip it. You'll just have to be 100% on top of your canopy management.
650W is on the low side, I don't like to be below 750W. Some cultivars like it at 750W, others need higher. 1,000W is generally the magic number. You'll obviously need better cooling with that but, to a certain point, you should blast them with as much light as they can take. More wattage means more transpiration which means you'll need more airflow to remove the humidity.
Keep the airflow high and the humidity low. Don't spray anything when it's close to lights-off time. OCD trellis them with the double layer you'd mentioned. Once the flowering stretch has stopped, prune anything that is more than 18"-24" from the top of the canopy as everything below that is an energy sink. Upgrade those lights. You'll be just fine!