I had three because I like children and had enough money to do so. If I hadn't had a bad birth with my third I would have been very tempted by four. Would never have had more than four as I wouldn't have felt capable of giving them enough attention.
As a society it seems to me that if some people don't have any children and others have one them some people will need to have three or four. So I would be in favour of benefits paid up to four children.
That said I am strongly against our present system where the tax paper steps in and absent parents aren't made to pay for their children.
At the moment child maintenance isn't deducted from benefits because it is considered too unreliable. This seems the totally wrong solution to me.