@PersistenceUnit versus @PersistenceContext

@PersistenceUnit(unitName = "myJPA")
EntityManagerFactory emf;
...
EntityManager entityManager = emf.createEntityManager();

@PersistenceContext(unitName = "myJPA")
private EntityManager entityManager;

PersistenceUnit injects an EntityManagerFactory

With EntityManagerFactory and @PersistenceUnit you should create/destroy EntityManager every time by hands and manage transactions too
and PersistenceContext injects an EntityManager.

EntityManager interface is used and instantiated directly. It has an internal mutable thread-local reference to a real EntityManager

Implementations of methods just redirect calls to this real EntityManager.

And there is a servlet listener, that before each request obtain EM by calling EMF.createEntityManager()

and assign it to that inner reference of special EM.

Also this listener manages transactions by calling getTransaction().begin(), .commit() and .rollback() on the real EM

for(int i=0; i < 100; i++){
   insert into table
}

Question is whether the above will use single connection (or) more connections.

The answer is below

A JPA EntityManager that is JTA managed will use the same JDBC/database connection for the duration of a JTA transaction.

So as long as your method is in a JTA transaction, your code will have the same connection.

Outside of a transaction, the container may acquire a new EntityManager for each operation,

Across every JTA transaction boundary the proxy will release its JPA EntityManager and acquire a new one (or at least clear it).