What is the JMX mbean to get Read count , write count on a table for cassandra version 3.11.4

You can extract it from the ReadLatency and WriteLatency metrics that have an attribute called Count that represents number of read or write operations correspondingly. The metric name is org.apache.cassandra.metrics:type=Table,keyspace=<ks_name>,scope=<table_name>,name=ReadLatency for reads, or org.apache.cassandra.metrics:type=Table,keyspace=<ks_name>,scope=<table_name>,name=WriteLatency for writes.

See Cassandra metrics documentation for more details.