Below sql will help you to find max connections allowed, connections used, superuser/normal reserved connections:
select max_conn,used,res_for_super,max_conn-used-res_for_super res_for_normal
from
(select count(*) used from pg_stat_activity) t1,
(select setting::int res_for_super from pg_settings where name=$$superuser_reserved_connections$$) t2,
(select setting::int max_conn from pg_settings where name=$$max_connections$$) t3