Dear H.,
I think it's a terrific idea to start using a sticker chart to help your children get motivated to engage in behaviors, chores, responses, etc that might be somewhat challenging.
The most important thing, even before you figure out how many boxes to fill, is to define in observable terms what you want your kids to do. For example "Behave" is a bit too unclear. Perhaps try something like, "Use nice words when talking to others" (It's just an example...not sure what you are working on with your kids). It's very important that your kids understand what your expectations are so that they don't feel confused and/or angry when they might not earn a sticker for the day. If you do this, you will be able to ask them, "Did you use nice words today?"
Giving your children a sense of empowerment and ownership also helps with motivation. Perhaps have your kids decide something that they need to work on at home with you so that it doesn't feel imposed.
As far as the number of boxes, stickers, etc...start out small. Another important, perhaps key feature, of any behavior mod plan is to ensure the child meets with success almost immediately. If the task seems too daunting, the motivation to persist might not be there. Also, the whole point of using a reward system is to get a behavior to become part of someone's repetoire of skills. In order for that to happen, make the periods to earn the reward longer and longer. Generally what happens is that the behavior becomes part of the child's daily routine and the "reward" isn't so important any longer. You might also try using non-tangible rewards such as special time with Mommy so that your house doesn't start to become buried in stuff. Another key component is that what the child is working towards is of great importance to them. If your kids don't really care about the reward, there's not much incentive to engage in the behavior you're looking for.
Hope this helps!!
Best,
D.