When do you use the word so and when do you use the world very, is there a rule.

For example: the food is very hot. or there have been so many different people walk through here.

Ok...I'll go ahead and stay with the examples you gave above.

Very and so can be used interchangeably in informal grammer. Both act as adverbs.

Very distinguishes a comparative degree often less than the superlative (i.e. not the hottest, just very hot food). Websters gives the definition as:
-adverb 1. in a high degree; extremely; exceedingly: A giant is very tall.
2. (used as an intensive emphasizing superlatives or stressing identity or oppositeness): the very best thing; in the very same place as before.


So can also indicate a degree of comparison, but is used more finite, somewhat exclamatory statement of superfluousness (not only were there very many different people, but there were SO many different people).
-adverb 4. to the extent or degree indicated or suggested: Do not walk so fast.
5. Informal. very or extremely: I'm so sad.

So to keep it simple, very is the formal way, so is informal.

Hope this helped!
Jon