Let S represent the # of known solar systems. Let P represent the # of planets discovered so far.
SP/ S+P= N
N = Number of Planets of an average solar system.
Solar systems in the average galaxy?
Let Ns Represent the average number of stars in a galaxy.
Ns*S= Ss
Ss is the total #
Galaxies in our (the) universe?
Let Z represent the average size of a galaxy.
Let G represent the known galaxies in the sky.
Let A be the size of the universe. A=15 billion light years.
Let E represent the size of the average void.
Let Ne represent the number of known voids.
A- (E*Ne)/ G-Z
and that will give you an average #.
Take all that I told you with a grain of salt, as the actual count could be way greater. This is just assuming.