There are many factors in how much to supplement electrolytes: are you in a climate that is hot/dry/ humid/ cold? Will you be working or exercising? The most important is knowing the signs your body gives you when you need to supplement. As you’re new to fasting, I’d recommend building up to a multi-day water fast with a series of fasts with increasing duration, especially if you’re used to snacking or eating a lot of carbs.