There is no ND64 for the Phantom as far as I know. ND32 is the second best and the best for multiple lighting scenarios, so I'm waiting mid july for the standalone filter from TACO.
It's simple, on a typical summer sunny day with 1/50 shutter you have ISO100 and F22, and because your P3 camera is fixed to f2.8, you need 6 stop filter to maintain the shutter. That filter is ND64. If you use ND32, you get 5 stops which is already giving you 1/100 shutter and everything fast is getting stutters. It's not that bad, but if you use ND8, it gives you only 3 stops so you're getting 1/400 shutter and that is really noticable, you want to be bellow 1/100, idealy on 1/50, so ND32 is the brightest one I would use. If there is a cloudy weather and you need eg. 4 stops to kill, it is better to go 1/25 with ND32 or crank up the ISO to 200, than 1/100 with ND8.
You CAN use all the filters in a set, if you have the time to change the filter every time the cloud passes, but I don't think anyone will do that. I'm aiming to do the highest possible production with the P3, and I think that is unnecessary, I will just take off the filter when there is not enough light...