They have too many children. (They can not provide for all of the children.)
They have very many children. (They have a large number of children.)
Too is used to express the idea of more than is needed, wanted, or desirable.
He is too loud.
She is too tired.
Very expresses the idea of a large amount or a high degree. Very is often used for emphasis.
He has very many CDs.
She earned very good grades.
She drives very fast.