Water heater sizing question

[Ted J.]

I am buying a 12 unit studio complex in Santa Clara, California. The current water heater is 75 gallon light commercial recently replaced. I haven't heard any complaints about not enough hot water. Out of the 12, 10 units have singles, the other two have couples. There is no dish washers in the units. The common laundry room has one washer and two dryers. 

The WH first hour rating is 134Gallons per Hour, then 81 gallons per hour with 90F temp rise. The DOE web site calculation seems right on the margin:  https://www.energy.gov/energysaver/water-heating/s..., assuming 10 showers, 6 shaves, 3-4 food prep in the peak hour. Does that cut too close? Anyone with experience on this?

Thanks!