NPI stands for "National Provider Identifier" and is a unique 10-digit identification number issued to health providers in America by CMS (Centers for Medicare and Medicaid Services (CMS). It is a simplification standard for the Health Insurance Portability and Accountability Act (HIPAA). Covered providers must share their NPI with other providers or any entity that may need billing purposes.
Your doctor should be able to provide this information.
Source - CMS