My wife has been breastfeeding our baby boy since birth. And milk demand has naturally increased over time as baby grows. How can she increase her milk supply or should we go for a mixture of milk powder feeding and breast milk feeding? Any advice or recommendation?

Your wife can try to increase her milk supply by taking milk boosting foods like oats, lactation cookies, fenugreek etc. Different mommies have different foods that work for them, for example, a friend of mine drinks milo with a teaspoon of coconut oil and there will be an increase of milk. If the milk boosters work and she produces more milk, then it's best to continue with breastmilk because let's face it -- breastmilk is incomparable; it has the best sources of vitamins and minerals for baby. However, if there isn't much difference in her milk production despite taking the milk boosters, then you can introduce formula as a supplement to the breastmilk. Lots of mommies do this and it works well too :) http://sg.theasianparent.com/how-to-increase-breast-milk-supply/