How many days should I do intermittent fasting?

Exactly what type of IF are you doing? It really doesn't make any difference for weight loss. If this is something that you will do for the rest of your life, then it is every day for the rest of your life. If you find an eating window fits into your lifestyle then good...personally I have never eaten breakfast so before IF was a "thing" I do it naturally, but that didn't keep me from gaining weight. The only thing that will help you lose weight is to limit calories. Logging what you eat is the best way to look at your diet.

Some kinds of IF inherently aren't every day, as you may know, such as 5:2 (eating maintenance calories 5 days, very low calories - maybe 500 - 2 days each week).

I went with zero days, personally, because - as a decades-long committed hedonist - it sounded unpleasant to me. If you want to try it, that's great: If it helps you manage your calorie intake and stay happy while doing it, it will help you lose weight.

It's still the calories that are the direct factor in weight management. More specifically, the average daily calories you eat over a week or few. The timing of the calories isn't terribly important, unless it affects your satiety or energy level, for example. (Those factors can affect our calorie intake and expenditure, so they indirectly affect weight management in that way.)

As far as how often to do it, you might think about your overall lifestyle. If you work out on certain days of the week, would fasting on those days improve your workouts, or hamper them? Do you not feel hungry in the morning so skipping breakfast is easy with no penalty later, or are you the type of person who will have raging appetite later in the day if you skip breakfast? It's OK to experiment, try out different fasting routines, consider how they work for you.

Most of this "eating preferences" stuff is very individual, I think. Other people can tell you about their experiences, but those experiences may not be the same for you.

Note: Some people feel that IF has certain health benefits, as a separate issue from weight management. I'm explicitly not commenting on that aspect here. You don't say what your goal is, in trying IF, so we're all pretty much assuming it's weight management of some type. If that's not true, making your goals explicit here might get you more nuanced advice.

Best wishes!