According to my own theory:
- TNG/DS9/VOY pre FC are all one universe (Prime) where Zefram Cochrane flew the Phoenix without Borg/E-E interference
- Events of FC created alternate past, where most of Cochrane's crew was killed during Borg attack, E-E crew helped him rebuild the Phoenix and he knows
some events of the future (that most likely had some effect on ship design decisions). This timeline creates new universe, let's call it "Enterprise" universe/timeline which leads to NX-01 being a thing (don't you wonder why we never heard of NX-01 before in Prime? And TATV is not canon, anyone who brings it up will be thrown out the airlock). At some point later in this universe USS Kelvin is built, then much later Romulus is destroyed and that's when Nero comes back in time and creates yet another timeline/universe, the Kelvin universe.
-STD forked from either Prime or Enterprise timeline, probably 50+ years before USS Kelvin was built (since we don't have any ships that are like Prime nor Kelvin universe)
So we have 4 universes/timelines (Prime/Enterprise/Kelvin/Discoverse) plus no reason to think that each one doesn't have a corresponding MU (so about 8 universes we know off)
And it's not that far fetched. Within Star Trek we have actually seen this: