Hi, ​@MSGA!

It will only run once for the contacts you already have.

If you wish to save every new contact, you could create an automation and use the "New contact created” trigger:

You could also use a Date/Time based trigger to get all the new ones that subscribed at the end of the day:

Hope this helps!

Thank you ​@Fabio Gaulke for the reply. Does this seem right (below)? 

Also, this is the broadcast I set up for that first run. Will I have to create another broadcoast for the automation to run? Or is that first broadcast “running” in the background already? Sorry for the stupid question maybe, just not really understanding the broadcast → automation relationship.

Thank you ​@Fabio Gaulke for the reply. Does this seem right (below)? 

 

Also, this is the broadcast I set up for that first run. Will I have to create another broadcoast for the automation to run? Or is that first broadcast “running” in the background already? Sorry for the stupid question maybe, just not really understanding the broadcast → automation relationship.

 

Yes, this will work. Just keep in mind Google's request limit for the Sheets integration. 

And now you have a trigger that runs this automation for every new contact created. As soon as you start this it won't be necessary to run broadcasts to save your contacts. You only need to run it to contacts created before you activated this automation.