Hola Nicole
Do you mean tanto here is an adverb? It is when we are comparing the "action", for example:
"Rosario bebe tanto como Carmen" (Rosario drinks as much as Carmen.)
In this case tanto is invariable.
But tanto is an adjective when we compare "things", for example:
"La gallina tiene tantas plumas como el gallo." (The hen has as many feathers as the rooster.)
As an adjective, here, it needs to agree with the noun (plumas, feminine plural).
Hope this helps
Saludos
Inma