Postgresql too many clients error on linux server

The only way to increase max_connections and persist this value is modifying the postgresql.conf file, so first of all, check if the value has changed (after restarting the server):

show max_connections

If the value DID NOT change, there's something wrong with your procedure (file permissions maybe?). If the value DID change, you might try with a higher value (weird, but might depend on your application connection requirements, OR a connection leak).