I totally agree with you, standinginthedoorway: arguably in the appalling situation of losing a child, it could be framed as even WORSE (if that were possible) to lose one if you have one other, as the loss of a sibling must be absolute agony too, plus you yourself as the parent have to somehow deal with your own grief and that of your surviving child... God, can't believe I'm even putting all this down in words but the OP's thread has brought all this to mind! Please God none of us ever have to be in theis awful position no matter how many children we may have 
I think in a way, crazy though I know it all sounds, the OP does have a strangely valid biological point, even if it sounds odd to be framed in this way. Surely in centuries past, part of the reason for having so many children (apart from merely the lack of birth control) was absolutely as some sort of insurance policy - child mortality was high and with no welfare state whatsoever plus a feudal economy, you would have needed a certain amount of children just to survive your old age yourself...?
Benzalkonium's point is an interesting one and did make my heart hurt for a bit on reading it as yes I can very much see that losing an only child would be a different experience (though no more awful, logically) than losing one of four, especially when you also have grandchildren. Obviously being left 'alone' like that, as a parent, must be devastating beyond comprehension. But they really are just two very different circumstances (partly because of the grandchildren thing) and also because (disclaimer: This is NOT to say that losing one of four children is just like misplacing your favourite socks or something; I cannot even imagine the agony of it) four is by any standards a big family, a very different feel from a family with two kids. So - if I'm right, OP - as you are only thinking of adding one more to your family and not three or four more, for example, I'm not sure, even by your own 'logic', that the situation of losing one of two would neccesarily be any less destructive a blow to your family life as losing one of one would be.
Oh, Lord, I'm tempted to delete all of this as it sounds so awful in print - again, I am not for a single minute saying that losing a child when you have many is easier than if you have only one or two. I had a young friend at school who was killed in a road accident when we were 12 and she was one of 4 children - I can't believe her parents' pain at losing her was lessened by having three other children still living :(
Let's all hope for the blessing of healthy children no matter how many we have!