1. There is now way to make a 51% attack impossible. Actually it's even proven that you need 2f+1 honest nodes (roughly 2/3) to guarantee progress. Dan avoids this by making synchronous assumptions.

However, the best way would be infinite witness votes to guarantee the highest overlap.
How many consensus nodes there are. It depends on the performance, less nodes > performance, more nodes < performance but potentially more security.

However, in DPOS the more nodes you have the more witnesses have to come to an agreement which gets more complicated the more there are (people are difficult).

So I honestly think 20 is a good number.