An element's atomic number is 25. how many protons would an atom of this element have?

An element's "Atomic number" IS the number of protons
in the nucleus of each atom of the element.

If an element's Atomic number is 25, then it must follow ...
as the night follows the day ... that each atom of this element
has 25 protons in its nucleus.  If it has a different number of
protons in its nucleus, then it's an atom of a different element.