It's 15. Looking at the newest wave of Campaign previews, some sites say the Locke canyony mission, titled [spoiler]Swords of Sanghelios[/spoiler], is two thirds of the way into the Campaign.
Other sites have said it's the tenth level. So my meagre arithmetic abilities tell me that puts H5 level with H2, ignoring intro cinematics and training 'level' Armoury, at 15 playable levels.
So, another H2 split? 8 levels with John and 7 with ... Wait. Was H2 13 playable levels?
H5 bigger than H2's space opera confirmed.
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But this all depends on the actual size of the levels and how long it takes to complete the story. I just hope the campaign is a couple hours longer than what it was for Halo 4.