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The thread pool used by openHAB is defined in "/etc/openhab2/services/runtime.cfg". Adding a line with "org.eclipse.smarthome.threadpool:ruleEngine=" will define the new number of threads used for the Rules engine. Rolling this out using Ansible, I use a "lineinfile" Task to update the configuration file:

- name: Set number of Java threads used for Rules
  lineinfile:
    dest: /etc/openhab2/services/runtime.cfg
    regexp: "{{ item.regexp }}"
    line: "{{ item.line }}"
    state: "{{ item.state }}"
  loop:
    - { regexp: '^org.eclipse.smarthome.threadpool:ruleEngine=', line: 'org.eclipse.smarthome.threadpool:ruleEngine=20', state: present }
  notify:
    - restart openhab2

Most likely this config entry will not be there in the beginning, then the default of "5" is used. Adding this setting, and restarting openHAB results in "20" threads. Verification by using "/usr/share/openhab2/runtime/bin/client":

openhab> config:list "(service.pid=org.eclipse.smarthome.threadpool)"
----------------------------------------------------------------
Pid:            org.eclipse.smarthome.threadpool
BundleLocation: null
Properties:
   discovery = 5
   ruleEngine = 20
   safeCall = 10
   service.pid = org.eclipse.smarthome.threadpool
   thingHandler = 5

openhab> shell:threads --list | grep ruleEngine | grep -v grep | wc -l
20

The system is more responsive now, but with added workload the "20" might not even be enough in the future.