Mucho gringo

I recall experiencing a slight gramamtical shock the first time I heard someone say mucho gringo. Why didn't they say muchos gringos? They mean the same thing, so why the difference?

• Hay mucho gringo en Puerto Vallarta. There are a lot of gringos in Puerto Vallarta.
• Hay muchos gringos en Puerto Vallarts. There are a lot of gringos in Puerto Vallarta.

Although I have given these two examples the same glosses, there is a difference. In the example with mucho gringo, gringo is being conceptualized as a homogenous, indivisible substance, like air or water. In the example with muchos gringos, the speakers sees gringos as consisting of a group of individuals.

More examples:

• Oye, con tanto chavo aquí, ¿no has visto uno que te lata? Hey, with so many chavos here, haven't you seen one you like?
• Ya sabes, puro ranchero. I overheard this at the supermarket. The speaker was complaining about a party where there were a lot of uncultured people.
• Pura niña fea. Nothing but ugly girls. (movie dialogue)
• Hay mucha maquiladora en el norte. There are a lot of maquiladoras (special factories) in the north.

As you can see, all but one of these examples refer to groups of people, often in a derogatory light. That makes sense, given that emphasizing group traits over individuality can be dehumanizing.