Fill data automatically in worksheet cells

Use the Auto Fill feature to fill cells with data that follows a pattern or are based on data in other cells.

Select one or more cells you want to use as a basis for filling additional cells.

For a series like 1, 2, 3, 4, 5..., type 1 and 2 in the first two cells. For the series 2, 4, 6, 8..., type 2 and 4.

For the series 2, 2, 2, 2..., type 2 in first cell only.
Drag the fill handle .
If needed, click Auto Fill Options and choose the option you want.

Source: Microsoft