Okay so you can use more or less marshmallows depending on taste. This isn't rocket science. It's Krispies, people!
You can use more or less cereal. Again, this is not rocket science. Do it to your own taste.
Krispies are best the day of. But that being said they can be eaten for a couple of days after you make them. Store in an air tight container or they go stale. Store stacked with waxed paper in between layers or they stick together.
Also microwaves are all different so you need to get to know yours. Get up close and personal with it so you don't mess up recipes by cooking things too long or not long enough. It pays to be friends with your microwave. Like all friends, microwaves vary, so adjust your cook time accordingly and keep one eyeball on it at all times while using it.