im running a gavitta 1000 de and 4" outtake f...

4-8? depends how big you want to get em and their pot size.

more plants means less vege time - up to a point. too many variables to give you an answer. 12 plants might be the most you can fit and ~2 week vege - maybe 3 weeks at most? this works out very well for your slightly rectangular shape. 1.2x1.2' per plant. 4 colas per plant will do well for this context.

And with leds that I'm using at the min I don't have an extraction fan or inlet fan just good circulation and some co2...

There not just hot I have a 4 inch burn on my back when the bulb stuck to me using an umbrella reflector and bulb hangs downward. The light penatration is so much better than led but the heat is going to be a problem. I used a closed system with another duct coming off the circulation pipe to an outside vent to use as a vacume to bring in the coldest air from outdoors.... hope that helps...

It depends on the expectations you set for yourself, more or less a 4x4, ballpark 100w per plant you want to grow, early on you would be ok but once the plant/s got to a certain size the tent would require a more powerful cfm than you could get from a 4" exhaust, not saying you couldn't but if you plan to grow robust plants and fill the 4x4, then the tent will have a large transpirational footprint and without proper ventilation you could see problems at critical moments in flower, again this relies on what you plan to grow. This holds true whether its 10 autos or 4 photos. DE reflector is about 4×6 ft (110x180cm) at 3 ft distance in overlapping plans, providing light levels of up to 1000 µmol s-1 m-2. HPS (High-Pressure Sodium) lights are considered very hot, producing a significant amount of heat compared to other grow lights, particularly when compared to LED grow lights; this is why they need to be positioned carefully away from plants to avoid burning them. Depends on your climate too, someone who lives in a warm tropical area will have different needs than someone who lives in a colder region but I'd certainly upgrade ventilation and possibly look into a cooling solution as I think HPS can run hot, 900W of LED are hard enough to cool in a 4x4 and if HPS is hotter id need some serious cooling. Again it is dependent on the goals you set for yourself and the growth you wish to achieve. Good luck with your new grow.

Try and fill the space with as little plants as possible. Nice big pots, soil is best in my opinion get to a good height, maybe week 5/6 put a net across and spread them out, continue to grow and tuck branches in net to keep all growth at 1 height while you flip to 12/12... Good luck 🍀 👍.