An atom has an atomic number of 17 and an atomic mass 0f 35: How many electrons does this atom have?

The atomic number is the number of protons and ...

The number of protons is always equal to the number of electrons in an element.

Hence if the the atomic number is 17, there are 17 protons and 17 electrons to match the number of protons

This atom has 17 electons

Yes 17 electrons, having 17 will mean that it has 17 electrons