depends on the predator. and how many you want. a scale would be between 30-60 ( if you count the really small sideburn dreads ) example, my Wolf has 32, my gen2 p1 has 62 including the really small ones.
length wise again is personnel choice, some like them long others more akin to the P1 and shorter, you can vary the length to give a more random look, rather than all the dreads the same length ( i mean you think predators have hairdressers? ) and i like mine to be longest at the back slowly getting shorter as the move forward towards the cheaks.