Someone we know has four children - two twins - which I've always considered a rather efficient way to go - two, rather than four, pregnancies and two, rather than four, post-natal hazes. But here's an even better option - kidnap some once they sleep through the night, are (mostly) toilet-trained, and can (mostly) feed and dress themselves.
We are enjoying our family of ten this weekend, although I can't imagine how mothers of eight keep up with them all as a full-time thing... I've spent all day making sure everyone is present and accounted for (with a bit of hand stitching in between, but no baking/cleaning/ironing - do I sound sad?).