The benchmarking is by default performed on a single GPU at a time, and if all GPU's are identical, there is no need to benchmark more than one.

It's recommended to define hashrates based on what a single GPU can perform. To display profit information for a complete mining computer, use the Group profiles in Options dialog, Profit profile section. A Group profile can define how many of each GPU (represented by a Profit profile) the system has. See Benchmark of algorithm hashrates for more details.