Mummies is the only way to make the noun mummy plural. Confusion arises because some people mistakenly believe that most nouns should form a plural ending in s. When a noun ends with [consonant] + y, you need to change the y to an i and add es to form the plural.

What is a plural form of mummy?

noun. mum·​my | \ ˈmə-mē \ plural mummies.

What is the correct spelling of mummies?

noun, plural mum·mies. the dead body of a human being or animal preserved by the ancient Egyptian process or some similar method of embalming.