Learn Swedish: Vad har ni för rosor idag? - What kind of roses do you plural have today?

Together, vad ... för means "what kind of" or "what sort of". In Swedish, this phrase is usually split up. Vad starts the question, and för comes later in the sentence, right before the noun you are asking about (in this case, rosor).

Why is för placed exactly where it is in this sentence?

Swedish questions must follow the V2 rule, meaning the verb (har) comes in the second position, right after the question word (vad). The subject (ni) follows the verb. Finally, för is placed immediately before the noun it modifies (rosor).

Why do we use the indefinite form rosor instead of the definite form rosorna?

When you use vad ... för to ask "what kind of", the noun that follows is always in the indefinite form. You are asking about a general category or type of roses, not a specific, known group of roses.

Could ni be used here as a polite, formal singular "you", or is it strictly plural?

While ni used to be a formal singular "you", modern Swedish rarely uses it this way. It is safest to assume ni is the plural "you". Here, the speaker is likely talking to the staff at a flower shop, addressing them as a group.