The current Mac Pro has six TB2 ports so it can drive three 5K displays if your willing to dedicate all six ports.
This assumes a 5K Apple Display would even have Thunderbolt 2 inputs...or that there would be some kind of dual-Thunderbolt 2 to Thunderbolt 3/USB-C adapter. All big assumptions at this point, not to mention the graphics impact that would have on the machine.
The cleanest external 5K implementation would be a Thunderbolt 3 spec that support DisplayPort 1.3 instead of the current 1.2 standard. Problem is Intel has no plans to support DisplayPort 1.3 until Cannonlake at the earliest so we're still a year or so out from 5K being a resolution that can be pushed comfortably without some kind of Frankensteinian solution either through MST (multi-stream transport) or some kind of eGPU setup.
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