It adds -(e)n in all cases except the nominative singular.
der Experte but des Experten, dem Experten, den Experten.
Not sure why those are called weak nouns but that's one word I've heard for that class of nouns.
How does one know that 'den Experten' here is in the singular or the plural?
Because einstellen takes a direct object in the accusative case.
den Experten can be masculine accusative or plural dative, but since we expect a noun in the accusative case here, it must be masculine (singular) accusative.
(plural accusative would be die Experten.)