She had three servings, just like elle est revenue deux fois implies elle est venue trois fois.

Elle s’est servie → elle s’est servie (au moins) une fois.

Elle s’est resservie → elle s’est servie deux fois

Elle s’est resservie deux fois → elle s’est servie trois fois

See also When is it okay to add "re" before a verb?

This means that the person had 3 servings:

Se Resservir means to Refill (not exact, but works with this)

So with this logic, it means that the person served themselves once, then they refilled their plate twice:
1 + 2 = 3

So the individual had 3 servings

In English, we say helpings of food.
She had three helpings of my quiche.

Or: She helped herself three times to my quiche.

When you have re for food, it means a second time. se servir = one helping, se resservir, two helpings.

But bear in mind, not every verb in French with RE means more than one. It can get pretty tricky. Just one quick example: Elle a remonté la rue……She went up the street OR She went back up the street. Only the context will allow you to decide which is right.

Perhaps a colloquial translation would be “She came back for seconds of my quiche twice”, thus implying she had 3 servings.

En toute rigueur, elle a eu trois parts de quiche.

Dans la vie courante, la répétition (resservi, deux fois) sert souvent à appuyer le discours et non à noter l’accumulation. Dans ce cas, il est fort probable qu’elle n’ait eu que deux parts de quiche.

On trouve aussi cela avec le verbe réitérer par exemple.