Set-feature for that value is how the host system indicates how many queues it is requesting. A subsequent get-feature returns how many queues were actually allocated. This can be less than the maximum supported by the drive, such as if the OS didn't request very many queues (due to you not having very many CPU cores), or if the drive is being virtualized and some of its queues are already allocated to a different virtual function.

That's the number of queues requested by the host system. If you ran that on a system with four CPU cores (and no SMT), you'd likely see that same drive return 4 instead of 7.