Try using the Drake equation with values that you find reasonable. How many civilizations do you estimate there are in our Galaxy?
Briefly, the Drake equation is used to arrive at a rough, probabilistic estimate of the number of intelligent, technologically advanced extraterrestrial civilizations within the Milky Way galaxy whose electromagnetic emissions are detectable.
The many variables include:
i. The average rate of star formation in our galaxy
ii. The fraction of those stars with planets
iii. The number of habitable planets per star
iv. The fraction of habitable planets on which life actually appears
v. The fraction of life-bearing planets on which intelligent life emerges
vi. The fraction of civilizations that develop a technology that releases detectable signs of their existence into space
vii. The length of time such civilizations release detectable signals into space
The estimations on these variables can range from reasonable to speculative given the lack of data sets to draw upon when it comes to the existence of life on other planets. Choosing what estimates feel right to you will depend in large part to your own inclinations. However, there are generally agreed upon ranges for each variable that can help guide your own calculation.
Given the most up-to-date research, the rate of star formation (i) is estimated to be about 1.5-3 stars per year. The fraction of stars with planets (ii) is estimated to be about 1 - meaning stars with planets are the rule as opposed to the exception. Based on data obtained from the Kepler space mission, it is estimated that the number of habitable planets (iii) in the galaxy is around 40 billion out of a total of about 100 billion, or 0.4. Trying to determine the fraction of habitable planets that actually develop life (iv) is where it starts to get speculative. Since we only have one sample, Earth, to draw from and life seemed to develop relatively quickly once conditions were right, this variable is usually set at 1. The fraction of life-bearing planets which develop intelligent life (v) is even more difficult to gauge. Scientists dispute to what degree our solar system's location is uniquely free of destructive forces and how inevitable intelligence is in evolution. Determining this value will depend a lot on your own inclinations. Those who feel it is inevitable, generally put the value at 1. The fraction of civilizations which develop the ability to send communication signals into space (vi) would depend on the likelihood that a intelligent life form discovers radio technology and intentionally - or even unintentionally - transmits them into space. Drake himself estimated this variable at 10%-20% or 0.1 - 0.2. Finally, one estimation for the overall lifetime of such a civilization (vii) has been developed by Michael Shermer. Shermer arrived at his estimation by calculating the lifespan of sixty historic Earthly civilizations. After doing so, his estimate settled at 420 years.
If using some of these estimates you could arrive at a formula that looks like this:
N = 1.5 x 1 x 0.4 x 1 x 1 x 0.1 x 420
N = 25 communicating civilization in the Milky Way