Given that the "percentage of stars with planets" part of the Drake equation has recently been determined to be close to 100%, Panspermia is starting to feel more and more likely.
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IMHO, "We don't know" is the only answer to the question of how many planets have life on them or the probability of some forms of live existing somewhere. 0 is as valid as 10^128 until more than one other life supporting planet or moon is found to establish some baseline for speculation. otherwise, we're talking sci-fi here, in which case I think stargate's version seems decent.
[1] Though maybe it does speak to life tending toward evolving more complex, energy consuming systems, and its propensity to spread out into the universe.