A pistil is composed of three filaments, naturally tied together at their base.
Saffron's finish will not be equally powerful, it all depends on the kind of recipe you are preparing.
These are the average quantities that are usually recommended, but you may change them according to taste.
Sweet or savoury recipes : at least 1 or 2 pistils per person (i.e. 3 to 6 filaments per person)
Savoury recipe containing sea food, or paella :at least 3 to 4 pistils per person (i.e. 9 to 12 filaments per person)
Custard cream or other milky desserts : 10 pistils (i.e. 30 filaments) per litre of milk
Cake : around 6 pistils (i.e.18 filaments)