score:4
This answer is helpful, let me repost the relevant parts.
USCIS says:
You do not need to apply to change your nonimmigrant status if you were admitted into the United States for business reasons (B-1 visa category ) and you wish to remain in the United States for pleasure before your authorized stay expires.
The only situation in which you need a B-1/B-2 visa specifically is if you want to use the same visa for different trips, of which some are business and other are purely tourism.