Place chicken in large pot and pour water over to cover.
Roughly chip 2 ribs of celery, 1.5 onions, and 2 carrots. Add to pot with chicken. Season with 2 tsp of salt and 1 tsp of black pepper.
Bring pot to boil over high heat.
Reduce heat to medium-low and allow chicken to simmer 1 hour, covered.
Remove chicken from the pot and allow to cool before removing and shredding the meat.
Strain the stock and discard the solids. Reserve liquid and add water if necessary to measure 12 cups of liquid.
Finely chop remaining celery, onion, and carrot.
Place pot over medium-high heat and add oil.
Add chopped vegetables, remaining salt, black pepper, and garlic. Cook frequently until vegetables are tender and onion is translucent.
Add reserved stock and chicken base and stir to combine.
Bring to boil and reduce to simmer. (~10 minutes).
Increase hat to high and bring to boil. Add pasta and cook, stirring occasionally 15 minutes or until pasta is tender.
Reduce heat to low and add chicken and season if desired.
Cook additional 5 minutes.
Stir in chopped parsley.
Serve with grated parmesan.