You can use strand channels. Use Generate Strand Data operator to create a strand channel with random values. Another option is to create a expression with Generate Strand Data percentage and use the expression languaje to create a strannel channel beased on a percentage value:

$baseValue=0.0000;#0.0,1.0

$strayValue=1.0000;#0.0,1.0

$percentage=10.0;#0.0,100.0

$result=hash($strandId) <= $percentage/100.0 ? $strayValue:$baseValue;

$result

This expression will create such channel. You can find the full explanation here("Stray Color" section. It is for Maya but it works the same on C4D).

In Maya this code will also create some UI controls to tweak the values but this is not supported by C4D (It doesn't support QT UI framework), so you have to tweak the values in the code: Tweak the "$percentage" variable, which value ranges from 0.0 to 100.0. As you can see the value in the code is set to 10.0.

I hope this is clear enough. 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)