Using MQTT with multiple subscribers

MQTT v5 has the support of shared subscriptions and mosquitto version 1.6 added support of MQTT v5.

Check release notes

Good article on shared subscriptions here

Typically you design MQTT applications in a way that you don't have overwhelmed subscribers. You can achieve this by spreading load to different topics.

If you really can't do that, take a look at the shared subscription approach sophisticated MQTT brokers like MessageSight and HiveMQ have. This is exactly the feature you're looking for but is broker dependant and is not part of the official MQTT spec.