How to prevent the Hibernate Connection pool to launch the connection limit exception?

Question

I have a stateless DAO using JPA that uses the Hibernate Connection pool to apply your transactions with the database. Whenever the entityManager Hibernate is started, I have the following line on my server console:

Hibernate connection pool size: 20 (min=1)

If I have more than 20 clients asking for transactions at the same time, Hibernate throws an exception on the connection limit number. I must deal with this problem manually or Hibernate has some functionality that allows you to stack orders and distribute free connections to customers on hold?