You have to work it out. The starting point is how long the shutter speed needs to be. Fast moving water like a fountain blurs quite quicky and you might get the effect you want at something like 1/8sec. Waterfalls a bit longer. Milky seascapes need at least a couple of waves to pass, so maybe 8-10 secs, streaky clouds depend on the wind, but a few secs. You get the idea.
So you lengthen the shutter speed accordingly, and depending on the light level, by raising the f/number and dropping ISO. Then if you still need more, add an ND filter. To cover all those subjects, you probably need a couple. I use ND16 which is four stops, and ND500 which is nine stops. YMMV.