The Drake equation is advocated by SETI (Search for Extra Terrestrial Intelligence) to show that the probability for ETI has a high probability (N).

N=N_*×f_p×n_e×f_l×f_i×f_c×f_L

• N_* Number of stars in the Milky Way
• f_p Number of habitable planets in each system
• n_e number of planets that are Earth like
• f_l number of planets that emerge from inorganic matter or organic precursors
• f_i fraction of those planets on which intelligent beings also evolve
• f_c fraction of those planets on which sufficient communications technology arises
• f_L fraction of average planet lifetime

The problem with the equation is that each f is a number between 0 and 1, the product of the equation will be vastly lower than the total number of suitable stars in the galaxy N*.  Many variables are unknown.  So, the numbers that are brought in depend profoundly on the assumptions we bring into the problem.