What is the difference of these four "mucho" muchos" muchas" mucha"? Do I say muchas gracias??
mucho, mucha, muchos, muchas
Each form of mucho has to match the thing it is describing in gender and number.
Mucha comida, comida is feminine and singular
Mucho tiempo, tiempo is masculine and singular
Muchas horas, horas is feminine and plural
Muchos libros, libros is masculine and plural
So "Muchas gracias" is correct.
They are all adjective forms that would match the noun that they modify in number and gender (not so when used as an adverb...me gustas mucho.)
Yes, muchas would match gracias in both gender and number.