Is it "a number of empirical studies have examined (...)", or "a number of empirical studies has examined (...)"?
Number can be plural or singular. Here, we'd use the plural. The studies are separate entities, taking place at separate times and in separate locations. This requires plural construction.
If the sentence said, "The number of studies is small," then number would be singular. It's not the studies that are under consideration; it's just the total quantity of them.