I'm only 22, and so I haven't had kids yet. Actually, my turn toward the MMM way of life is because of my desire to build a really healthy and stable family! I admire MMM for having gotten to retire before having kiddos, and would love to be there to commit myself to raising them from day one. So, I'm working hard now to make up for it. :)
That means my post is going to be a little different - more from the perspective of a kid than of a parent. Some background:
I'm an only child, but my husband is one of nine. I was sent to private school etc, and my husband was homeschooled (uh.. More on that later).
My parents ran a business and easily made $250k a year or more throughout my younger years. Unfortunately, they were both "spend it while you got it" types and burned it all. Some of it, they burned on private school and gymnastics and fancy toys - I mean, I had the barbie pool, and every single video console up to the Playstation 2. We had personal computers in the house, in the 1990's - plural computers. We went to the ballet, and we went to the Bahamas, Florida and Hawaii. I wanted for nothing, ever, and that's great. But did any of that extravagance benefit me? Maybe the traveling did, especially to the Bahamas where the American culture was a lot less; Maybe the computers did, because they got me into tech early... But they weren't things I needed. I didn't need a computer at 8 years old, and I didn't need to go to the Bahamas at 12 years old.
One of the biggest things that is noteworthy in this story is that I was sent to private school as a little kiddo. I was never ever asked IF I would go to college, but instead WHAT I would go to college for. But, when I turned 17, and my parents and divorced and moved on from having a family and planning anything for me, no one had any help for me. My mother is buried under a pile of debt, and my father announced to my face that I was worthless and went on to peruse other endeavors. I haven't gone to college because of the lack of financial support, and am having to educate myself and make my way in the world in other more creative ways... It really sucks to be fending for myself.
On the other hand, my husband was the young-middle of nine kids (Number 5 of 9). His father was a molecular biologist who worked as a post office clerk for $40 an hour; his father was also bipolar, and it's pretty awesome he could hold down a job and all his kids survived. His mom stayed home and made as many babies as possible (no exaggeration, none of the kids had more than 9 months between them, and those that did were due to miscarriages, most only have a handful of weeks between their conceptions...). She also decided to (attempt to) home school all of them, but only got through the first 3-4 before she completely gave up. But she still didn't send any of them to real schools, even after running into this failure in health and patience. Mom passed on in the late 2000's and dad passed on in the early 2010's. Their house had 5 bedrooms, and was well and thoroughly trashed by the time I met my husband. It was the kind of house where you would walk on the other side of the road to stay away.
No one ever got anything new, all toys and clothes were hand me down, and education went downhill quickly after the 4th and 5th child. No one had the time or the patience. After the deaths of the parents, my husband was barely 18, and was left with a portion of life insurance, and a portion of the value of the house he grew up in... That came out to about $30,000, a GED he worked on his own to get, and nothing more. The siblings fight like cats and dogs, so my husband just doesn't talk to any of them. It's more painless that way. It really sucks for him, too, to be fending for himself.
So, all that said:
TL;DR How many kids you have isn't nearly as important as what you do with and for them.
As long as you're not running around willy-nilly with your head up your butt not paying attention to what is going on - financially and emotionally - everything is going to be fine. Your kids will come out fine. Educate them, however many you have, and give them a foothold when they become adults.
But, I still wouldn't have more than 4-5 kids. It's probably a bad idea to go with any more than that. Things can get very bad, very fast.
Just my history and two cents. :)