Valency refers to valence electrons, which are the electrons that are physically localized in the outermost regions of the atom, and therefore the highest in energy and the most available for bonding or interaction with other atoms and forces. Valency is a measure of what an atom can or is most likely to do in terms of its chemical properties.
Valency for the first 18 atoms or so is a pretty straightforward consideration. Electrons are only "allowed" to arrange themselves in particular locations and orders around an atom, generally called orbitals and shells. When an inner, lower-energy shell is filled, electrons must go to the next available one, which is farther from the nucleus and higher in energy. Therefore, valence electrons will be all the electrons that are found in these "outer" shells, ignoring any electrons that are in "filled" shells. The innermost shell can only hold 2 electrons, but increasingly more are available as the elements get heavier. The easiest way to evaluate valency is to look at the periodic table, and simply count how many places the element is located from the left side of the chart. This is a little less accurate for the transition elements, but if you're still learning about valency you can deal with those weird exceptions later.
Since sodium is our example (symbol Na) we can see that it's one step from the left side of the chart, and therefore has one valence electron.
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