To be able to determine how many bonding electrons a molecule has in sigma bonds and pi bonds, take a look at its Lewis structure.
So, to determine how many of each you have, look for single bonds, which are actually sigma bonds, double bonds, which contain 1 sigma and 1 pi bond, and triple bonds, which contain 1 sigma and 2 pi bonds.
The Lewis structure of phosphorus trifluoride looks like this
The central phosphorus atom is bonded to three fluorine atoms via single bonds, and has a lone pair of electrons attached.
The phosphorus atom is
As a result, phosphorus trifluoride has 2 sigma bonding electrons for every single bond present, for a total of 6 sigma bonding electrons, and zero pi bonding electrons, since it doesn't form any pi bonds.