And bear in mind that these can also be redeemed for flights in F.
AFAIK, it's not possible to redeem topbonus miles for F awards. Moreover, availability of C awards is not so good as I could see from their online tool.
Is there a limit for the number of passes that you can buy? I don't see one...
Yes, it's limited to one. Point 2, last sentence:
Ein Kunde darf innerhalb des Aktionszeitraumes nicht mehr als einen Citytourpass erwerben.