Confirm your Business Unit Unsubscription Settings.
They should be set to "Unsubscribe from this BU only".
This is assumed in the following points.
a) If you are looking for people that are currently set to Status "unsubscribed" because of an unsubscription in a certain timeframe:
You can run a basic
"SELECT fields of your choice
WHERE status = "unsubscribed"
DateUnsubscribed between '2019-10-01' and '2020-3-31'"
query on _Subscribers inside the BU in question. It will only return people with an Unsubscribe Status for the BU that the query runs in.
For this usecase, you can also make use of the _BusinessUnitUnsubscribes data view in Parent and have the BusinessUnitID as a filter criterion. That way you don't need to run the same query multiple times. You do NOT have to filter for status here, as only people with a status of "unsubscribed for this BU" make it to the Data view to begin with. If they resubscribed in the meantime, they won't show.
Which leads us to b:
b) If you are looking for unsubscriptions that occurred (events) during a certain timeframe, and it does NOT matter whether the relevant recipient still has the status = "unsubscribed" now (e.g. due to resubscribe in the meantime, bounce in the meantime):
In this case, the _Unsubscribe Data View is Business Unit specific.
This means: You can simply run a query against _Unsubscribe without "MID = 12345" (which is not a valid field) and the result will be all Unsubscribe Events in the BU the query runs in in the timeframe of your choice.
be sure that you go down the right path a) or b): ask yourself whether you need UnsubscrIBERS (people) or UnsubSCRIPTIONs (Events).
Then just run the query in the right BU, or possibly make life easier in option a, with _BusinessUnitUnsubscribes.