We have a car seat in each car for Master M and the one booster for Master J but when this bubs is born we are going to need to get another 2 seats as bubs would be reward facing and Master M will still be in his car seat as he only weighs 11kgs.

Ideal it is upto you! if you think it would be to much hassle to keep swap and changing then get the 2 if you can afford it!