Hi, I know i am answering to this post after a long gap still it may help to others.
As per your post even after you increasing the number of connections from 64 to 128 you are still facing the connection issues.
Here first we need to check all the opened connections are closed or not if these are not closed then the issue may occur even if you increase it to 252.
Go to CM -> Impala -> Charts Library -> check "Active Frontend API connection"
Monitor the cluster if many users are using the cluster or not if you think connections are high and users are not using the cluster then the opened sessions/connections are not closed after the use.
set the "idle session timeout" in the Impala configurations, by setting this if a user opened a session and performed any operation and leave the session it will become idle session and this session will be closed by setting this timeout.
Same if you are running the impala queries from Hue you need to set the session time out something like below in the Hue Service Advanced Configuration Snippet (Safety Valve) for hue_safety_valve.ini
As like if you are using ODBC/JDBC clients you need to set the timeouts at the client side.
If you would like to check the number of connections/sessions those are Active and inactive Go to CM -> Impala -> click on any Impala daemon -> Go to Daemon WEB UI -> click on "\sessions" tab