Hmm...well, the sort of anti-benefits family I grow up in (and a bit of common sense) tells me people on low incomes shouldn't really be having 12 kids then expecting state handouts... (Although it'd be unfair to the children to push them into poverty because their parents thought "na, condoms are too expensive we'll have another one") but other than that, no I don't really think there should necessarily be limits. Except perhaps in countries that are developing fast, it has potential - what China's One Child Policy did without a doubt was prevent an exploding population becoming a supernova. Although it caused significant problems.
Eugenics? Sadly it's a bit inhumane...and I'd fear if it was adopted widespread I'd be deemed not one of the good ones xD
My geography studies have led me to come to one conclusion though...if there were to be a set guideline by governments on how many children to have it would be 3. Replacement rate is 2.1, but accidents happen so...3's safety.
Hope I'm not going to get referred to the CIA as a Nazi or anything - I know we're not Facebook, they probably just hack PC anyway xD