No rules except what is feasible within the publication method and schedule. There is a case for page turns though. A uneven amount of pages can mean it either starts or ends in a spread. Shonen Jump actually use this to have more control over the magazines layout, which is why they can promise "center colors" or "opening color spreads" for series some times. It also propels the reader more easily since he/she can just keep on flipping.
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