How can I apply a value to multiple cells in Numbers?

For something like this you have to understand how Numbers (or any spreadsheet) sees your data.

Conceptually, every row represents a distinct set of data. I this case some kind of call sheet for various actors.

Now, visually we use tricks such as borders, spacing, colors to logically group chunks of data - such as the call sheet per day on set, but that means nothing to Numbers.

In order to categorize the data you need some common link. Since you want to organize the data by day, every row needs to have a day/date value. Otherwise, as far as numbers is concerned, rows 6 and 20 from your initial screenshot are the same day since they contain the same (in this case, blank) date value.

Stick with me, though. While it may seem like more work up front to add the date to every row, there are other advantages to it.

As an example, I recreated a snapshot of your data as a single table:

Certainly not as pretty, but that's just the raw data. Now we can get Numbers to do the heavy-lifting.

You want a breakdown by day (either Shoot Day or Date), go to Inspector -> Organize -> Categorize and say so:

In two clicks, Numbers categorizes your data based on the shoot day:

Easy. But we're only just getting started.

Maybe you need a cultist by agency so you can send each agency a list of actors that are needed. Change the Categorization to Agency: and maybe add a subcategorization for day, so each agency gets a list of which actors they need to send for each day:

Or maybe you want a cultist by actor, so you can clearly see when each actor needs to be on set:

All this is leveraging the power of Numbers to organize your data, separating the main 'data entry' table from the final visual output that's more useful.

I think you will find your last approach is by far most efficient. Also copy DAY1 DAY2 etc down into the cells below and eliminate the blank rows so you have the data in true tabular format that can be easily sorted, summarized, etc.

I recommend having a look at the Categories feature and at Pivot Tables.

Intro to categories in Numbers on Mac - Apple Support

Intro to pivot tables in Numbers on Mac - Apple Support

This may not look exactly like what you were originally thinking, but you will like the ease and efficiency in analyzing, summarizing, and presenting your data.

If you need to present to other people you can then easily copy paste into a "report" table and format that to taste.

SG