* Definition of "professor": Different countries have different academic ranks and titles. What constitutes a "professor" in one country might be a "lecturer" or "associate professor" in another.
* Types of institutions: The count would differ depending on whether it includes only university professors, or also those in colleges, community colleges, technical schools, etc.
* Data collection challenges: Accurately collecting data from every university and institution worldwide is a massive undertaking.
While some organizations may attempt estimates, any number you find would be a rough approximation at best.