you can never have enough onsies. i put the short sleeve ones under everything for mercie during the winter. especially under her p-jams during the winter since babies don't keep their blankets on and even with the heater on it is still cold. and during the summer she practically lived in them around the house and she slept in them during the summer. plus, so many onsies are so cute now, that i would but the cute print ones to go under mercie's overalls as t-shirts. i used them for her until she was about a year and a half or so.
at least a million. I like short sleeves for summer - long sleeves for winter. I think the best deal is carters since you really layer things over thm anyway - they dont have to be super fancy.