I will cover your question on my following notes over the next days...

The quick and dirty answer is: the more the better.

Yes, even for only 2 S9, you will have serious downtime in late autumn and winter when the sun barely show up... Even if you've got a roof full of panels (I have 17 kwp). I'm located in central Europe btw, near the Coast of North Sea.

I'll be able to give you some statistics from my first 180 days of mining. It's mostly estimation though, only hard facts are mining rewards 😉

But my data is getting better because I'm logging accurate data on Power Consumption and Hashrate per day via Graphana in Home Assistant since January.