How to increase max pool size in ActiveRecord?

I get the error:

Error "ActiveRecord::ConnectionTimeoutError - could not obtain a database connection within 5 seconds.  The max pool size is currently 5; consider increasing it."

How do I increase the max pool size?

# DB CONNECTION
DB_CONN = ActiveRecord::Base.establish_connection(:adapter =&g开发者_如何学运维t; "sqlite3", :dbfile => DB_FILE)

config/database.yml
pool: 8 (default is 5)

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A static number of connections can be specified, but ...

# config/database.yml
default: &default
  adapter: postgresql
  pool: 99 %>

.. but if the number of connections needed is proportional to the number of threads, it may be desirable to specify it dynamically. In this example, two connections per thread.

  pool: <%= 2 * (ENV.fetch("RAILS_MAX_THREADS") { 5 }) %>