Basically handler counts are the number of threads which is responsible to process the request for hadoop RPC server.
Best practice is to set the RPC handler count property dfs.namenode.handler.count to 20*log2(number of datanodes) with an upper limit of 200.
If you have 12 datanode then number of threads should be - 20*log2(12) = 80 (dfs.namenode.handler.count = 80) max you can increase it to 100.
If you have specified 140 then all the handlers will be active to process the request.
You can verify it by taking a thread dump of namenode process and check the total number of handlers and what they are doing.
You can take a thread dump by running $ kill -3 <namenode_pid>
This will take a thread dump of Namenode process in .out file under /var/log/hadoop/hdfs (Namenode log directory.)
Another method is to check the Grafana Dashboard (If you are using Ambari ) and check the RPC queue metrics for Namenode.