how to configure the number of processes BIND uses?

Solution 1:

You need to edit the relevant startup script (based on your system) and pass in -n <number_of_processes> to bind (or named).

-n #cpus
        Create #cpus worker threads to take advantage of multiple CPUs. If not 
        specified, named will try to determine the number of CPUs present and 
        create one thread per CPU. If it is unable to determine the number of 
        CPUs, a single worker thread will be created.

Solution 2:

Edit /etc/sysconfig/named and set the OPTIONS variable, e.g. the below limits BIND to 8 threads:

OPTIONS="-N 8"

Editing the relevant startup script does not seem to work these days (2021).