I would have answered to OP
- “what do you LIKE to do?”, as erg cz also suggested.
Ratting in carrier in null provides me around 100Mish/h. But it is very boring (as ratting is in general, doing the same anomalies again and again).
I can accept that people focus on isk/h if they have a goal to reach and need isks for that.
After one hour of ratting i have to stop, and prefer to do some explo. At least i have some fun. - How many accounts does one need ? it depends where you live and rat with your carrier (null sec block area? etc), if you have to defend your carrier regularly, if you accpet to lose it sometimes or prefer to drop a fax to rep it, if other people can provide you cyno etc.
- the best carrier/super carrier? i suppose here it is only for pve. So you can read this very good guide, now a bit outdated but which contains valuable infos : Carrier Ratting Guide
Supercarrier is a real different thing: much more expensive (30B ish with fit/implants etc), mure more juicy target, you need a keepstar to dock it, and good alliance mates to save you when you get tackled by 50 wormholers guys, so… - the least skill intensive? no idea