According to parallel universe theory ( which as far as I know is not any different from string theory/ies ) anything that can happen will happen, if not here then in any other, parallel universe.
If "ANYTHING THAT CAN HAPPEN WILL HAPPEN IN ANY OF THE ALTERNATE UNIVERSE" were to be true it simply means that the either the no. of alternate universe is determined by the probability of the occurrence of an event or the probability of occurrence of an event is determined by the no. of alternate universe; cause each of possibility "HAS TO OCCUR".
But none of the above can be true as if "no. of alter. universe" is determined by the " no. of possibilities of occurrence of an event " than that would mean as for each and every "event" there r different no. of "possibilities" of "different outcomes" ; so for each and every "event" the "no. of parallel universe" will be "different", that does not seems to be true.
If we take second case that the "no. of para. universe" is "fixed" or even if increasing, than it is increasing according to some rule ( no matter how complex it be. ) and "no. of "possible outcomes" of different probability" is determined by them, than that would simply mean that "each and every EVENT" has Same probability ( or at least utterly related ) but for an "infinite" no. of events ( may be hypothetical ) to have same or completely related "probability of "no. of "possible outcomes" of different probability" " is surely way out.
So all these things doesn't simple say that this idea of " ALTERNATE ( or parallel ) UNIVERSE " should be ridiculed or this idea must be abandoned that " "Anything that can happen will happen in ......." .