Select value in drop menu to populate adjacent cell

You can consider using the SWITCH function. Compact and easy to edit.

=SWITCH(A2,"apples",3,"pears",4,"bananas",10,"peaches",8)

In this example A2 is the Pop-Up Menu. The value in B2 changes automatically when I make a different choice in A2.

After the A2 in the formula you simply have a series of switch-value if-match pairs.

More on SWITCH here:

SWITCH - Apple Support

Use ; instead of , in the formula if your region uses , as the decimal separator.

SG

"…so I select apples from a drop menu and the adjacent cell populates with "3" or some value."

You will need a set of cell pairs to tell Numbers what value is to be placed in the target cell for each of the items listed on the pop-up menu. These can be placed in the main table, or, as done in the example, on a separate table

Example:

Each cell in column B of the main table (Table 1) contains the same pop-up menu list of nine fruits:

The second table, "Items", contains the same list in column A, and a list of numbers, each associated with one of the named fruits.

Each cell in column C of Table 1 contains a copy of this formula:

When a fruit name is chosen in a cell in column B of Table 1 gets that name for the cell on its row of column B, searches for that name in column A of Items, and, on finding the name, returns the value in the adjacent cell of column B of Items.

Regards,

Barry