The atomic number of an element tells you the.. most abundant isotope.
number of protons.
number of protons plus neutrons.
mass of an element.

→ number of protons.

Explanation:

let's take an example of hydrogen:

hydrogen has atomic number 1, because it has only one proton

a saline water is fed to a reverse osmosis unit at flow rate o rotameter connected to reject (retentate) line reads 8 installed in reject and permeate line reads 15.2 milli siemens/cm and 300 micro siemens/com respectively.