What is something in spanish you could say if someone said mucho gusto?

The most common and appropriate response to "Mucho gusto" (Nice to meet you) in Spanish is:

El gusto es mío. (The pleasure is mine.)

You could also say:

* Igualmente. (Likewise.)

* El placer es mío. (The pleasure is mine.)

* Mucho gusto en conocerte. (Nice to meet you too.)

These responses are all polite and show that you are happy to meet the person.