How many summons do you need on average to get a banner hero?

On a Banner there's an 0.80065% chance per pull. So the chance for a first pull out of a total number of pulls is:
10 pulls 7.7%
20 pulls 14.9%
30 21.4%
40 27.5%
50 33.1%
60 38.3%
70 43%
80 47.4%
90 51.5%
100 55.2%
110 58.7%
120 61.9%
130 64.8%
140 67.5%
150 70.1%
160 72.4%
170 74.5%
180 76.5%
190 78.3%
200 80%

And at 2K per pull you can do the math from there.

I'd only pull on special banners or spend 12k vis for the +chance% on the 6th pull.

If it's not a limited unit, just buy their shards in the shop during their banner and wait to get them down the line.

Depends, am I Senturion or not?

It's so random and there's controversy over so called tables, and posts about people spending 70k on WOL or other limiteds and not getting them. I got 4/6 out of the last UR units within 4 pulls but I never got the ones in particular I wanted, I'm inclined to say if you don't get it within 24k you're probably not going to get it, and its so random and low chance plans are worthless, just react with an open mind.

20 ish

Maybe 30-40k vis.