Correct me if I am wrong in understanding this "Using a pgbouncer-per-webserver we easily got to 20k.", but does this mean say you have 5 web-servers, each web-server has an instance of pgbouncer installed? or you have 5 web-servers with 5 separate pgbouncer servers (1:1 mapping between webserver and pgbouncer) while all pgbouncer server transact with a powerful, single master Postgres.