There are two bearings in each wheel of a scooter. Bearings for scooters are classified according to their "precision", also referred to as their rating: ABEC-3, ABEC-5, ABEC-7, ABEC-9. Bearings for scooters are similar to bearings for skateboards. They may be open or closed, which is important for their durability, since an open wheeled bearing takes in more dirt and water.
Most scooters you buy already have a fitted scooter bearing. However, if you want to change the scooter bearing, it is relatively simple on a plastic tire wheel, but it may be a more time consuming for wheels with aluminum cores. We recommend watching a video on how to do this correctly before buying bearings for an aluminum wheeled scooter. If you buy bearings for a scooter, which does not already have bearings, be sure to buy a "spacer", otherwise you may damage your bearings.
You can read more about bearings in this guide for scooter bearings.