How many clients can it hold?

Tens of thousands of concurrent connections on ordinary server hardware, and several thousand on a small ARM board. The published benchmark report on the download page gives the exact figures and the conditions they were measured under.

Will my existing MQTT clients work with it?

Yes. It is a standard broker: anything that speaks MQTT 3.1.1 or 5.0 — Paho, mosquitto_pub, Node-RED, Home Assistant, ESP32 firmware, industrial gateways — connects without modification.

Can I move over from Mosquitto or EMQX?

Yes, and clients do not need changing. What has to be recreated is the configuration: users, ACLs and bridges live in the dashboard rather than in a text file. Delayed publish is deliberately compatible with the EMQX $delayed syntax.

Does it need a database?

No. It uses SQLite in a single file next to the broker. A database is only involved if you choose to keep credentials in MariaDB or MySQL, and that is optional.

Can I authenticate against my own system?

Yes — point the broker at an HTTP endpoint of yours, and it will ask that endpoint whether a client may connect and what it may do. JWT tokens are supported as well, for devices that already carry one.