Well, we do not know for certain. If recent events are any guide, there are some things that we do now know that seem to suggest there could be lots of them! Back in the 1980's, the IRAS satellite discovered that perhaps as many as 40 percent of all stars seem to have 'excess' infrared emission which you could ascribe to dust and asteroidal material in orbit around them. Where there is smoke there is fire, so this means that stars prefer to have 'stuff' orbiting them, probably left over from the era when they first formed. The Hubble Space Telescope imaged the center of the Orion Nebula and also found that something like 40 percent or more of the young stars had circumstellar disks orbiting them: the perfect birthing environment for planets.
The recent spate of discoveries in 1996 of large planets orbiting nearby stars brought us to a new era in searching for other solar systems. We are now PERMANENTLY and INCONTROVERTABLY in a new age where other solar systems beyond our own are a reality, not speculation. Moreover, the spectroscopy performed on several massive 'brown dwarfs' shows they truly are failed stars and are close to being massive planets with deep absorption bands of methane. This means that by extension, some of these massive planets could be very similar to Jupiter and Saturn. If these large planets are detected, there could very likely be smaller ones we cannot now detect with the existing technology we are using.
To find Earth-like planets, we have to find something about the same size as the earth, orbiting at about the same distance, or in the save zone where Earth-like temperatures would be found around the parent star. If 40 percent of all stars have 'stuff' around them left over from birth, and if 5 stars out of 100 in the current survey have easily detectable giant planets, then there is no reason not to expect that there should be many more out of 100 nearby stars that do not have smaller planets that are just below our ability to discern. Suppose that out of 100 stars, 40 had planets, say, bigger than the moon. In our solar system, we have 9 major bodies with Earth and Mars at the right distance, and with the right mass. I would say that this suggests that for every 100 stars, there could be 40 x 2/9 = 9 stars out of 100 that have at least one Earth-like planet. In the Milky Way, with 200 billion stars, that's a LOT of earth-like planets ( about 20 billion!!!).