Planning for second child?

Hi parents, my Lo is 17 months now. Me and my husband is thinking to have second child, maybe not now, well anyway if we have it, it will take 9 months to come out :) but question is, what age gap is good for my Lo to understand that he is having a sibling soon and how do you manage having 2 or more kids as honestly we both is pretty occupied with having one at the moment. Thks!