U can grow any number of plants under lights it’s all depending on size of area ur trying to cover with ur lights. Like @blackthumbbetty asked how big the area u trying to grow in?
30 to 40 watts a sqft , no cheap blurples (I have some of those). Remember your lighting the space and the size of the plants dictates the space. You don`t need much to start, but for flower maybe even a little more than 40. There are plenty of DIY threads here.
It depends on what type of LED. They all use electricity to create light. It’s not like any of them will halve your bill. Plus, you have to buy new lights, and the best leds aren’t cheap. I’d guess at least a 10%-15% savings.
30-40watts per sq foot, depending on which lights you buy. This also ties into question #4
IMHO, any LED panel on amazon that throws a purple light. Especially the ones that tout 1000w for $129