There could be a multiverse and there are some universes within the multiverse where the people within them don’t believe in the multiverse, but not believing doesn’t mean it isn’t real.
Scenario B:
Conversely, there could be only one universe and the people in that sole universe believes in the multiverse, which in this scenario, doesn’t exist.
Two Scenarios:
Scenario A:
There could be a multiverse and there are some universes within the multiverse where the people within them don’t believe in the multiverse, but not believing doesn’t mean it isn’t real.
Scenario B:
Conversely, there could be only one universe and the people in that sole universe believes in the multiverse, which in this scenario, doesn’t exist.