The atomic number is the number of protons in the nucleus of an atom of an element. As such, it is a positive integer starting with #1# for Hydrogen then up to #118# for Ununoctium (a.k.a. eka-radon). Only a few atoms of Ununoctium have been synthesized and detected.
Elements #119# (Ununennium or eka-francium) and #120# (Unbinilium or eka-radium) are hypothesised and various attempts are being made to synthesize them. Their predicted lifetimes are very short.
In 1959, when the heaviest known element was Nobelium (atomic number #102#) Tom Lehrer wrote a song "The Elements" (to the tune of "A Modern Major General") naming all of them. It ends with a typically Lehrer style rhyme: "These are the only ones of which the news has come to Harvard, and there may be many others but they have not been discovered" (pronounced "dis-car-vard").