Well you won't have to restart at all really. If you really want to get through a chapter without losing any of your units then you're just going to have to take that much longer in planning out your strategy. I usually don't restart that many times but instead I spend a considerable amount of time on each of my turns making sure that I've taken all factors into consideration prior to moving a single unit. Better planning > less deaths, although there's still luck in the equation too.