Is a duplex receptacle counted as one or two outlets for branch calculations?
There is a limit on the number of plugs for non-residential projects.

NEC 220.14(I) Each yoke is counted at 180 VA.  If there is one outlet, two outlets (most common) or 3 outlets it is still counted as 180 VA.  If there are (4) or more outlets then it is counted as 90 VA per outlet.

What you are describing, a double gang box, with (2) duplex outlets will equal 360 VA.  Having a total of (3) pairs of duplex outlets will be 360 * 3 = 1080 VA.  Since each duplex is 180 VA it can also be counted as 180 * 6 = 1080 VA.

The long power strips that you are used to with 10 or 12 outlets is commonly called plugmold.  In the NEC it is referred to as a fixed multioutlet assembly.  Per NEC 220.14(H) the load depends on if the plugs get used simultaneously or not.  Simultaneous use equals 180 VA per foot of length.  Non-simultaneous use equals 180 VA per 5 foot length.