Toolkit uses threads to carry out analysis of features such as age and gender in parallel. The number depends on the number of faces to be analysed and the number of cores in your machine. If you are planning to analyse multiple people at once, we advise setting the threads to 1 per core.
If the number of threads is set to 2 (by default) the Toolkit will analyse 2 faces simultaneously (using 2 separate cores) and then move onto the next face. For a minor speed bump you can try using threads equal to the no. of cores in your machine but not more (setting higher nr. of threads can have a reversed effect).
The maximum is 16 threads per Toolkit instance. This, however, does not mean that the Toolkit can only analyse 16 faces at once but that it can process 16 faces in parallel – given that you have 16 cores in your processor (i7 processor has between 4-6 cores).