I read an article recently that said the average according to self reporting was 25-30 pounds. And something like 35-40% reported gaining 50 pounds or more.

So money and health aren't issues?

I'd eat my way to 600 pounds or as large as possible and still mobile. I want an apron belly that bumps my thighs with each waddling step.

I don't want a live in feeder, but sharing a living space with other gainers would be nice.

BMI is mostly flawed because we're misusing it. It isn't designed for assessment of an individual in mind. It's designed to give information on a community.
The higher the average number the lower the overall health of a community.It doesn't take into consideration things like muscle because it's not a diagnostic tool, it's a statistical one. The higher your number the more likely certain medical issues are to be present. It isn't perfect, but is generally a fair indicator of public health.

I've had it a couple times as I've gone through various levels of fat.

Being told that a 42 was as high as khakis went in Sears at that time.

First time I couldn't sit straight in a restaurant booth. Doubly so because it was at a buffet and I hadn't gone through the line yet.

Went to a friend's niece's birthday party and the place had a turnstile you went through to get in. I was too wide even sideways and had to go through the handicap one.

And my personal favorite one. This was just after I passed 400.
I went for my first appointment at my allergist and they didn't have a scale with a high enough limit to weigh me. The sliding weight could only go to 350, their digital scale went to 400 so all they got was ERROR.

I've done it twice.
First time was as I was hitting 220ish. It was back in the late 90s and finding above a 42" waist locally was difficult outside of big & tall shops. It was daunting to think about needing to go to a special shop for your size. Didn't actually slow my gaining any, but I heavily considered it. Luckily my local shops upgraded to carrying up to 46 and 50 as I started needing those.

My second time was my first time hitting 400. I was really out of shape and walking around and using stairs was getting exhausting. So I quit trying to gain and worked on conditioning as my weight settled back around 330. Working on getting back past there again.

Let's check my stats:
Height: 5'10"
Weight: 389 pounds
BMI: 55.8
Body fat: 53.8%
Ideal BMI weight: 174 pounds
Amount overweight: 215 pounds

Chest: 61"
Waist: 65"
Hips: 56"

Any other measurements you want?

Honestly there isn't a set number of calories before it's wasted. It is possible to take in more than you can process, but your own metabolism and body will effect that. It's just kind of trial and error til you find the point of diminishing returns on your calorie intake.
That said, unless you're a competitive eater you probably aren't going to have the capacity to hit those caps for a while. I wouldn't worry about it too much unless you're going hard on the gainer shakes.

I'm definitely the fattest in my family. Probably by 200 pounds. My dad used to make it only about 40 pounds but he passed a few years back.

Among my friends I'm still the fattest, but there are other 300 pounders.

My last job had coworkers bigger than me, there were a few over 500 and 2 were closing on 600.

They're weirder than that. The ear wings are part of kip's world that show off a mix of mental state and social standing. So like popular people will have large, full wings while outcasts might only have a few scraggly feathers.

This really depends on genetics and how your weight is distributed.
I'm naturally belly heavy and passed a 40 inch (101.6cm) belly just over 200 pounds (90.7kg) and I'm within a couple centimeters of you height wise.
If you're more pear shaped it could be higher weights.