OTRS has a max session configuration that is associated with the maximun number of connections allowed by the DB. We have found that increasing the max_conn setting in the DB (We use Mysql) does what is required. So i am pretty sure you can do the same in PG.
OTRS is limited to 100 by default, but you can increase that in sysconfig in Core::Session AgentMaxSessions, but I believe that limitation is the same number by default in the DB. So if you increase it, then change it in the configuration of you DB. In our case we have increase it to 500 in sysconfig and to 700 in DB
Best Regards.