It’s very hard to predict how likely, because we don’t have any experimental evidence. All we know for sure is this universe exists. It’s very hard to make predictions from just one data point!
There are lots of theories that do predict other Universes. There might be some evidence that continuous inflation theory might be true. If that’s true, new Universe’s with slightly different laws of physics are always popping into and out of existence. I guess we’ll have to wait and see if any evidence for that mounts up!
As David says, this is a great question, but one that’s very hard to answer! In order to make anything other than wild speculation, we have to think where the presence of parallel universes could have any measurable impact on something in our one. The main places that they could do that are, frankly, in areas of physics where we don’t know already that they don’t. (I think that argument makes sense). This brings to mind a few places: the early universe, near the big bang, and perhaps by affecting, in some small way, the properties of nature on the smallest scale. It’s not an area I personally know much about, but it’s really fun to ask these questions!