I suggest a call to FEMA or a private insurer in your area to aks what coverage is available in this situation. If coverage is available for each unit, then I think you are required to consider the $750k as the maximum amount of available insurance for your coverage determination. If individual coverage is not available, then the coverage limit is obviously going to $250k.
I also think you will have to place some coverage on each unit. See Q&A Part II, Question 7.
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The opinions expressed here are personal and do not represent opinions of my employer.