Is God's mercy limited to being one hundred parts, and will disbelievers not attain it on the Day of Resurrection, making it reserved for a limited group, and is questioning its vastness considered a sin?

The mercy attributed to Allah the Exalted is of two types: an attribute inherent in Him, uncreated; and a created mercy, numbering one hundred. Allah sent down one mercy to the world for all His creation and reserved ninety-nine mercies to bestow upon His servants on the Day of Judgment.

Disbelievers receive mercy only in this world. As for the Hereafter, Allah will deal with them according to His justice, and they will abide eternally in Hellfire, because they departed from the realm of mercy to obstinacy and rebellion against their Creator. If they were to enter Paradise, it would be contrary to Allah's justice and wisdom.