You should make separate nodes, because combining them would either mean one HDD failure and you lose everything or you need to set up RAID and would lose usable space. However, as you already saw in the estimator, it’s going to take a long time to fill 20TB. I suggest starting with one node and wait until it’s vetted. After that you can start 1 or 2 more. You can always add more once the first few start filling up. But there is not need to spend the power on keeping the other HDD’s spinning until they start filling up a bit.
That’s not what it says, that says after a year you would have about 4.84TB stored total. Not per month. At current traffic, it’ll take 5 years to fill up 15TB and you may never reach 20TB. One node or more node doesn’t matter as incoming traffic is shared on the same IP subnet. So you don’t get more incoming data if you run more nodes.