I'm having hard times understanding the equation of Drake, i can´t find the number of the variable that refers to the stars with planetary systems (fp)
For the purposes of the Drake equation you may as well assume that every star has a solar system.
At present, the exact fraction is unknown since the search techniques are limited to finding planets with certain characteristics. For example, transit searches tend to find close-in, giant planets; doppler shift surveys are also most sensitive to massive planets with short orbital periods, and so-on.
One can try and account for this incompleteness, but although some examples have been found, we are in the dark about the fraction of stars that have an Earth-sized planet at distances further than the Earth is from the Sun, or the fraction of stars that have any kind of planet orbiting beyond where Saturn is in our solar system.
For the Drake equation, you need to know the fraction of stars with planetary systems times the number of "habitable" planets per system is. There have been attempts to estimate this from Kepler data.
Possibly the best estimate at present is from Petigura et al. (2013), who estimate that 22% of sun-like stars have an earth-sized (1-2 Earth radii) planet that receives between 0.25-4 times the radiative flux of the Earth.
Obviously this is a lower limit because it doesn't include planets smaller than the Earth. It also doesn't include the moons of giant planets, which may also be habitable.
We still know very little about this fraction when it comes to M-dwarf stars, which are the most common type of star in the Galaxy...