I am using Mosquitto broker to implement MQTT protocol. But I am unable to find how clustering can be done in case of mosquitto brokers. Also is there any limitation on number of clients those can be served with one broker.
You can't do clustering with mosquitto.
Some other MQTT brokers out there which support clustering, including HiveMQ. HiveMQ has an elastic cluster ability with auto discovery and a distributed masterless architecture and works very well on cloud providers like AWS or Azure.
You can see a full list of all brokers which support clustering here.
Disclaimer: I'm a developer of HiveMQ, so this answer may be biased.