We have 3.
DW would love another, as she thinks our family would be more balanced, as right now it always seems as though one of the younger 2 is left out by the others.
The real reason though is that she loved being pregnant, and loved having someone totally dependent on her. Youngest is now at juniors, and at every milestone (school, then beavers/rainbows, then juniors, then cubs/brownies, and i'm sure again when they go up to secondary school), she really struggles with the thought she'll never have that again.
I love our unit, but it is a struggle. as PPs have mentioned, it's logistically more difficult going away etc. financially it's massive - especially once they're all school age (though this is partly our choice, as we always encourage all 3 to do a lot outside school).
for me though, the biggest reason for not going for another was due to DW's PND. it got worse/lasted longer after each previous birth. her (and me) going through that and potentially worse again, terrifies me.