> Is it better to create multi databases or create multi schemas?
You're missing one option imho: One database, one schema.
> I am writing a program that can have multi instances. Sort like a finanical
> accounting system that can have multiable companies. Where each company has a
> different name but the tables are an exact match to each other.
Then you could simply add a "company-id" column to every table (and make this column part of the primary key of each table). And if you run multiple instances of the program within _one_ company, then you can also add an "instance-id" column (and make this column also part of the primary key of each table).
Sincerely,
Wolfgang
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