JI No1 is used for potting up small, young plants. When they need repotting in a larger container, you then need JI No2. A year or so later, you should repot the plant into a larger container filled with JI No3 and it won’t need repotting again for a couple of years.
Soil-based composts are good choices for established plants because they are rich in nutrients and retain moisture well. But they are dense and heavy and drainage is not too good, making them the wrong choice for seedlings and containers like hanging baskets.
Be aware that John Innes composts do contain some peat, if you are trying to avoid it.