I lost my product key number for my office 2021

No, the full Office 2021 product key cannot be viewed in Office or online in normal cases.

For Office 2021, the usual path is to reinstall using the Microsoft account that the license was linked to:

1. Open any Office app if it is still available before the recovery.
2. Go to File > Account.
3. Check the email address shown after Belongs to. That is the Microsoft account associated with Office.
4. After the PC recovery, sign in to account.microsoft.com with that same Microsoft account and install Office from there.

If you try to enter the old key at setup and see This product key has already been used, that means the key was already redeemed and is no longer needed. Install Office by signing in with the linked Microsoft account instead.

If Office was bought from the Microsoft Store and has not been installed yet, the product key can be viewed in Microsoft Store Order History by selecting Install Office.

If the product key card was damaged, but there is valid proof of purchase, Microsoft support may be able to replace the key or provide other options.

References:

• Using product keys with Microsoft 365 | Microsoft Support
• Where to enter your Office product key | Microsoft Support
• Reinstall Office from the Microsoft Store app | Microsoft Support