The atomic number of rubidium is 37. The atomic number of an element represents the number of protons found in the nucleus of an atom of that element. It is a unique identifier for each element on the periodic table.
In the case of rubidium, its atomic number of 37 means that a neutral atom of rubidium contains 37 protons in its nucleus. The number of protons determines the element's chemical properties and its position on the periodic table. Since atoms are electrically neutral, the number of protons also corresponds to the number of electrons orbiting the nucleus in a balanced configuration.