how do i auto fill

Here's the relevant section from the Numbers User Guide for Mac (which you can also access and search via Help > Numbers Help in your menu):

Autofill cells in Numbers on Mac - Apple Support

SG

Hi ioannis,

The key here is the small yellow circle that appears at the middle of a cell boundary when you bring the pointer near that cell. Use the pointer to grab that Fill Control, and drag it in the direction away from the centre of the cell, and you will fill the cells through which you drag it with values based on the contents of the original cell.

If the first cell contains the name of a month, those in the drag path will receive the series of names of following months.

Other date values will increment one day for each cell receiving the value.

Numbers are a bit different. You'll need numbers in two adjacent cells to establish the size of steps in the list of numbers that are sent to the succeeding cells. Enter 0 and 2, and the series will continue with 4, 6, 8, 10…

For the two cases in your 'lets say' example, you'll get the results shown in the example below.

The cells filled with green are the seed cells for each example. The fill colour was added after the rest of the cells in the column were filled. Had it been placed prior to doing the drag, the green fill would have been included in all of the filled cells (except cell B2, which was not a seed cell or a cell that was included in the fill.

The yellow-filled Fill control can be seen on the bottom boundary of cell F2.

Regards,

Barry

Hi ioannis87,

this should give you an idea how it works

https://www.youtube.com/watch?v=ygVZYEwIIjE

Regards Ralf