I'm obsessed with odd numbers, so I always said 3 or 5. My oldest was an easy, good natured baby and I was thinking "definitely 5". Then ages 2 and 3 hit.
DD1 and DD2 are three years apart, and that was wonderful spacing. It took us a year+ of TTC both times before we got pregnant, so we started trying expecting another year and got pregnant right away! So DD2 and DD3 are 2 years apart and that spacing plus third kid has really kicked my butt.
We decided after DD2 that we would be done after 3 kids. God has reaffirmed that decision by giving my a really hard pregnancy, a preemie, and a challenging newborn (so far - she's 3 months old).