Trusting God to plan your family almost always means having more babies. Being fruitful and multiplying (increasing in number) the race of men necessitates having more than two children per couple on average (replacement fertility is 2.1). Numbers are indeed part of the equation, as is obvious from the negative effect of family planning (the western world is below replacement level fertility). But from this, those who disagree with us draw a false conclusion that we hold that if an individual couple does not have "lots" of babies they are not trusting God.
Trusting is often most difficult when one desires a child but God witholds the gift. The archetype of this trust (faith!) and fruitfulness is Abraham, who had one child by his wife, and that in his old age. Abraham trusted that he would be the father of many nations even when he took up his hand to sacrifice his only son. Abraham believed God, and it was accounted to him as righteousness. Faith is nothing less than trust beyond all reason.
Trusting God to plan your family may mean twenty children. It may also mean none. In this respect (the individual), numbers are irrelevant. Nonetheless, we affirm as Scripture does, that barrenness is a curse while a full quiver is a great blessing. But only God knows how many arrows you need in your quiver.