Question

How many valence electrons does Na have?

Answer 1

Sodium has $1$ valence electron.
Explanation:
Your tool of choice here will be the Periodic Table of Elements. Grab one and look for sodium, Na. You'll find it located in period $3$, group $1$.

The thing to remember about main-group elements, which are those elements located in groups $1$, $2$, and $13$ $18$, is that their group number tells you the number of valence electrons.

In your case, sodium is located in the group $1$, which means that it has $1$ valence electron. This valence electron is located in sodium's third energy shell.