How does the number of electrons change as you move from left to right across a period?

The number of electrons increase as you move from left to right if you are talking about neutral atoms with a charge of 0.
For example, lithium, on the left of the periodic table has 3 electrons (2 in the first shell and 1 in the second). However neon, on the far right of the table has 10 electrons (2 in the first shell and 8 in the second.)

Fine out you self hx