Ramcharitmanas tells that 31 arrows flew simultaneously and 10 cut the heads of Ravana, 20 cut his arms and 1 hit the navel whereas Valmiki Ramayana tells that a single arrow hit Ravana's chest.
As per Ramcharitmanas(Lanka kanda):
चौ- खेंचि सरासन श्रवन लगि छाड़े सर इकतीस ।
रघुनायक सायक चले मानहु काल फनीस ।। 102 ।।
Drawing the bow-string right up to His ear the Lord of the Raghus let fly thirty-one
shafts, which flew forth like the serpents of Death.
दो- सायक एक नाभि सर स्रोषा ।
अपर लगे भुज सिर करि रोषा।।
लै सिर बाहु चले नाराचा ।
सिर भुज हीन रुंड महि नाचा।।
धरनि धसइ धर धाव प्रचंडा ।
तब सर इति प्रभु कृत दुइ खंडा।।
गर्जेउ मरत घोर रव भारी ।
कहाँ रामु रन हतौं पचारी।।
One arrow dried up the reservoir of nectar in the navel, while the rest struck his ten heads and twenty arms with impetuosity. The arrows carried off with them all his heads and arms, while the headless and armless trunk danced on the battle-field. The earth sunk under the weight of the trunk as it rushed violently on, till the Lord struck it with His arrow and split it in two. While dying he shouted with a loud and terrible roar: Where is Rama, that I may challenge and slay him in battle?
As per Valmiki Ramayana (Yuddha Kanda):
स वज्र इव दुर्धर्षो वज्रिबाहुविसर्जितः |
कृतान्त इव चावार्यो न्यपतद्रावणोरसि || ६-१०८-१७
That arrow, which was inviolable as a thunderbolt hurled by the arms of Indra and irresistible as Yama the lord of Death, fell upon Ravana's chest.
स विसृष्तो महावेगह् शरीरान्तकरः शरः |
च्छेद हृदयं तस्य रावणस्य दुरात्मनः || ६-१०८-१८
That arrow, released with great speed and which was capable of destroying the body, tore off the heart of that evil-minded Ravana.