So, I always start each campaign with only a single lance of light and medium 'meks. It's tough, but doable. You will lose your first few contracts, because you don't have the manpower to scout and deal with any scenarios that pop-up. Once you hit two lances things become a lot easier, because you can scout with one lance and fight scenarios with the other. However, you do need to cherry pick your contracts and what scenarios you fight.
Three lances is what we generally recommend for new campaigns, because it gives you two lances you can play with and then a third to scout, or act as reserves.