Nominative / Accusative / Dative / Genitive of the interrogative pronoun wieviele Nominative  
Adjectival interrogative pronoun  
Wieviele Menschen werden kommen?
= How many people are going to come?
Substantival interrogative pronoun  
Wieviele wissen es?
= How many know about this? Accusative

The Accusative is need if the question goes after a direct object or the preposition requires an Accusative. There is no difference between the substantival and the adjectival interrogative pronoun.

Adjectival interrogative pronoun
Wieviele Kuchen backst du?
= How many cakes to you bake?
Wieviele Bücher hast du gelesen?
= How many books did you read?
Substantival interrogative pronoun  
An wieviele hast du gedacht?
= How many did you think of?
Für wieviele hast du gekocht?
= For how many did you cook? Dative

Adjectival interrogative pronoun  
Von wievielen Kuchen hast du eine Probe genommen?
= Of how many cakes did you take a sample?
Von wievielen Büchern konntest du dir den Inhalt merken?
= Of how many books could you remember the content?
Auf wievielen Stühlen sitzt du?
= On how many chairs do you sit?
Substantival interrogative pronoun

Wievielen gibst du Geld?
= How many do you give money?
Wievielen würdest du vertrauen?
= How many would you trust? Genitive

The Genitive of wieviele is theoretically possible but nobody uses it. Usually a preposition would be used.

Adjectival interrogative pronoun

Das Geld wievieler Leute hat er gestohlen?
= How many people's money did he steal?
Von wievielen Leuten hat er Geld gestohlen?
= The money of how many people did he steal?
Substantival interrogative pronoun?

Das Geld wievieler hat er gestohlen?
= The money of how many did he steal?
Von wievielen hat er Geld gestohlen?
= Of how many did he steal money?

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