To discuss quantities and amounts, we use muy and mucho - the two are NOT interchangeable!

Yo estoy muy mal. - I'm very sick.
Está muy cerca. - It's very near.

Muy has only one form regardless of number or gender.

Mi abuelo es muy alto.
My grandfather is very tall.
Mi abuelita es muy baja.
My grandmother is very short.

Mucho has four different forms because it must agree in gender and number with the noun.

singular: mucho (m) - mucha (f) - a lot of, much
plural: muchos (m) - muchas (f) - a lot of, many

Let's look at some examples of proper use:

Tengo mucho trabajo. - I have a lot of work.
Hay mucha pizza. - There is a lot of pizza.
Yo estudio mucho. - I study a lot.
Tienes muchos amigos en París. - You have many friends in Paris.
Ella tiene muchas uvas. - She has many grapes.
La clase de Matemática tiene muchos números.
Mathematics class has lots of numbers.
Hay muchas iglesias en esta área.
There are many churches in this area.