When you select an environment, Open Space/Cubicles/Offices/Concrete translates into a numerical number (between 2 and 4) used during calculation. This is a rough estimate of what you would expect the interference level to be for that region, and then the APs planned are going to be calculated against that number. If you think you won't have much interference, then 'Open Space' is the way to go.
There's no best practice document for planning, but I typically run 2 scenarios:
1 -> What's the lowest level of interference I'd expect to see for that region
2 -> What's the highest level of interference I'd expect to see for that region
This gives you the minimum and an ideal maximum number of APs needed to get the specified data rate using whichever AP type. The actual should be somewhere in the middle to account for outside forces not considered in the calculation. Especially if you expect a particular area to have a mixed interference level (which I'd normally draw as a separate region).