Everybody has their own magic number and different reasons for reaching it. My sister quit one shy of the planned 4 due to a difficult pregnancy. Another sister was totally dismayed to find herself pg with #3, two was all they could afford. I had a friend say she didn't want the kids to outnumber them. Another friend said two hands, two kids. I know people who don't think they could love another one as much as they love the one they have. Or are afraid of how to have two. Some people quit because of age. Some just never get pregnant again. My neighbor couldn't conceive, was told it was impossible. Went through IVF, got quads. At 1, she got pg. Wasn't using birth control b/c she didn't think it was possible. 3 months after having that one, birth control failed and she had #6. They both took permant measures to prevent a 7th.
As for me, I had a horrible rotten pregnancy my first time. It was awful. 9 months of morning sickness where I thre up on the hour every hour. I was on limited activity. I couldn't drive, I was having black outs due to the way he kicked nerves. His kicking broke 3 of my ribs. I went through 24 hours of labor (hard labor, I don't count the pre-stuff). My life was on the line and he went into distress. I had emergency c-section that left me in horrible shape. I couldn't even stand up straight for two weeks. During this time, I was planning the second.

He was the worlds best baby--slept through the night before he was one month. I wish I knew how lucky I was and could have appreciated him. The payoff with the pregnancy was worth it.
My second pregnancy was worse--much, MUCH worse. And I had a 2 year old. I was on bedrest. I kept going into premature labor. I was supposed to have a vbac, but wound up with emergency c-section #2. He came down with RSV at 2 weeks and had many complications which has kept him quite sick. He almost died several times. At two, he still doesn't sleep through the night. He is beyond active and keeps me running. He can be quite needy. He doesn't understand the word "no" and comes up with amazing things. A cat in the freezer wouldn't even cause me to blink. To call him "high maintance" is being so exteremly nice. EVERYONE knew I was done. Anyone who has a child (either attitudeor illness--and we had both) and/or pregnacy like that quits. Except me. I knew I was having a third.
Pregnancy #3 was much more like my first. Still not good, but good for me. I have another 4 month little angel, although not as good as my first. And I'm pretty sure I'll have that fourth.
I don't feel outnumber at all. DS4 sits on my side, and DS2 and DS4 months share my lap. When we go out to eat, DH and I sit across from each other, next to one child and one in the middle. We accept out children for who they are and work with their personalities. I think we realistic going into it, which helps a lot.
I will have that fourth close in age (no bigger than a just 3 year age difference). I too long to be out of certain stages and find having the two so close in age is better than the 3 year age gap I have is so much nicer. Oh well, I can go on, but I wouldn't. This is long enough.