There are several levels of ND filters, starting from ND4, ND 8, ND16 and so on. Each multiple indicates how many stops will be lowered when using the filter. The higher the ND level, the less light will come in, the longer the shutter speed you can use, the wider the aperture. The movement in your photos will become smoother.
Usually for scenes the ND filter used is at least 10 stops or ND 1000. By using this filter, you can use a shutter speed of more than 30 seconds. Meanwhile, for waterfalls, to get soft photos you can use ND8 (3 stops) and 10 stops (ND 1000) to soften the movement of waves in the sea.
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There are also 6 stops (ND64) which are widely used for photographing waterfalls, sunsets and sunrises on mountains, beaches or even deserts. Apart from that, 6 stops are also widely used for urban photography when you want to remove people from your photos.
One thing that is quite important, when using an ND filter, make sure your camera does not move due to using a long shutter speed. You can use a tripod if you don't have a stable place for a long time.