No, it is not possible to veg your plants 12/12, 18 hrs is optimal if you have an indoor environment (they will start to bloom automatically when light gets below 14 hrs outdoors). I did not see any benefit with 24 hrs, I think the plants like a little rest. 12/12 is for blooming only.
The reason people say they can get 4 to 5 per plant is because they are vegging the plants longer, and using soil. In other words bigger plants and less of them in an area. In a hydro system, you will not get that much per plant, but you will have many more plants.
Most all the replies to your questions are incorrect. Make sure you double check before you follow any advice.