Short answer: Once.
Longer answer: Reschedule if you feel you're not ready, especially if it's the first time. Makes no sense to do an interview and waste your and the consultancy's time if you're not at your best. Candidates reschedule all the time for all sorts of reasons, so if you didn't consider it before, there is this option. The earlier you do it the better for everyone involved.
Note of caution -- if you reschedule the second time, you should have valid reasons as it may be held against you. You will lose credibility.
Think about it like a first date. If (s)he cancels once, fine, maybe something happened. The second time... Well, (s)he's probably not that interested, or has other commitments/issues/...