Which of these two sentences are correct?
- Wissen Sie, um wieviel Uhr wir kommen an?
- Wissen Sie, um wieviel Uhr wir ankommen?
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Wissen Sie, um wieviel Uhr wir kommen an.
Wissen Sie, um wieviel Uhr wir ankommen?
This question is not so much about word order, but when to separate the prefixes of a separable verb and when not.
The simple rule is: separate the prefixes if the main part of the predicate verb is in V2 position. That means, the second element of the sentence. This is the normal case in declarative sentences:
Wir kommen um siebzehn Uhr an.
Note V2 means the predicate verb is the second element, not the second word, and there's also zero-position elements. As German predicate verbs are either in V2 or end-of-sentence position, this shouldn't worry you too much.
Denn heute kommen wir um siebzehn Uhr an.
Denn wir kommen an.
I think the latter is the counterexample you are after. But remember, the main part of the verb is in V2 position in that last example. That's why you have to separate the prefix, unlike in your first example.