I've found that the sector AI handles itself OK, I started playing after the worst of the sector AI bugs were fixed, apparently.
However, I tend to only ever have 1 sector, and I jam all the planets in it that don't fit in my core. I try to keep the best worlds my core worlds, but sometimes the general layout doesn't permit that exactly.
I tend to ignore the sectors for the most part, unless I find myself with excess materials. Then I may spend some improving sector planets. I find giving the materials directly to be inefficient, as the sectors spread it around all the projects they have planned, so you give 200 minerals and get 66/100 for one thing, 66/100 for another, and 67/100 of a third. Instead I just build the thing directly.
I find when the sectors start actually stockpiling materials, I usually tend to have a ton of materials as well, and I usually have plenty of influence to spare. So then I'll occasionally pay the influence for a big chunk of resources.
What sectors are bad at is prioritizing. They'll develop a 1 mineral tile before a 4 mineral tile because the 1 mineral tile is closer to the top-left (or whatever they start from). The AI seems to just look for the first spot for the resource they want, rather than the best. So manually building for them can help overcome this. They are also bad at pop assignment, seemingly, but I don't know what to do about that. I tend to just have 1 or 2 pops, but I assume this is a bigger deal for some people.
So I can say for sure there's no planet limit for sectors, at least not one you could reasonably reach. And the sector AI seems to do better with more. Also, having just 1 sector means you only need 2 governors, one for core and one for the sector. Not sure what benefits could exist for multiple sectors, aside from not having to be connected.