Argon has 18 protons. Its atomic number is 18, and the atomic number of an element is the number of protons in the nucleus of each of its atoms. A neutral argon atom therefore also has 18 electrons, with the configuration [Ne] 3s2 3p6.
Argon is a noble gas in group 18 and period 3 of the periodic table, with a relative atomic mass of 39.95. Almost all natural argon (about 99.6%) is the isotope argon-40, which has 22 neutrons alongside its 18 protons. Argon makes up 0.94% of Earth’s atmosphere, making it the third most abundant gas in air.
Source: Royal Society of Chemistry, Periodic Table: Argon
Image: Vial of glowing ultrapure argon by Jurii, CC BY 3.0, via Wikimedia Commons.