The number of Internet of Things devices by 2020 is forecast between
20+ BILLION devices according to Gartner and 38 BILLION according to
MIMIC MQTT Simulator supports simulating up to 100k (yes, 100000)
sensors from one host. You can generate any message for any sensor type
with any frequency to test your brokers and subscribers. Any number of
hosts can be controlled to provide unlimited scalability.