Does PgBouncer create a separate pool for every database config entry?
Yes, pgbouncer creates a separate pool for each database. You really don't want a pool size as big as 250 in a scenario like this - because that is 250 connections from pgbouncer to the postgres server for each database.

The max of 480 is the total number of connections from clients into pgbouncer, regardless of which database they go into.