When you refer to a plural noun, why do you not change the verb to 3rd person plural? Example: Habrá muchos papeles en mi escritorio.

The verb haber is used in the third person singular for there is or there are or there will be in the case of your example.

 

In Spanish, you use hay/habrá,hubo,había etc. whether the verb is followed by one item or many, unlike in English.

See lesson Using hay = there is / there are in Spanish