¿Cuál es correcto? Ella tiene a muchos amigos. O Ella tiene muchos amigos.
Personal "a" or no personal "a"?
Ella tiene muchos amigos is correct because there is nothing specific. To use the personal A in this case we would say , ie: Ella tiene a sus amigos Gerardo y Consuelo en casa.
The a is not necessary in your case because as JC pointed out its purpose is to introduce a specific, known person or people as the direct object of a sentence - usually done with a name but could also be a title.
Vi la casa. I saw the house. (Not a specific or known person, no personal a).
Vi a Maria. I saw Maria. (Specific, known person needs the a.)
Vi a mi abuela. (Specific, known person needs the a).
Ella tiene muchos amigos.
Most textbooks that I work with state that you usually don't use personal "a" with the verb tener.
Julián Chivi brought up an excellent exception, however.
more about tener this is from thought.co EXCEPTIONS TO AN EXCEPTION After tener: The personal a is used after tener when it is used in the sense to physically hold someone or to have someone somewhere. Tengo a mi hijo en los brazos, I have my son in my arms. Tengo a mi hija en el pesebre, I have my daughter in the crib.
The personal a can also be used after tener when its usage suggests a particularly close or emotional relationship. Cuando estoy triste y necesito hablar, tengo a mis amigos, when I am sad and need to talk, I have my friends. But tengo amigos, I have friends.