Hey @daisy welcome to the community.  I don’t think there’s a way to do that automatically. 

What’s the reasoning to have it sent out over days to a subset?

One idea is that you can separate it out by creating segments and splitting it out depending on a certain criteria (or export and import with a custom profile property to split it) then create campaigns for each.

If this is a new list and you want a subset a list to warm up the account, there’s a random sample feature in Klaviyo to get a subset of your subscribers:

https://help.klaviyo.com/hc/en-us/articles/115001145931-How-to-Take-a-Random-Sample-of-a-List-or-Segment#:~:text=To%20take%20a%20random%20sample%20of%20a%20list%2C%20first%2C%20head,Members%20(for%20a%20segment).

Thank you for spending time with me. The reasoning to have it sent out over days to a subset is I want to see how the campaigns performed in the previous 100 emails. If it succeeds, it won't take me long to create a similar campaign for the remaining 400 or 500 emails, if that campaign fails, I can fix it in time and send these remaining emails.

Sounds like going the random sample route might best for this scenario then:

https://help.klaviyo.com/hc/en-us/articles/115001145931-How-to-Take-a-Random-Sample-of-a-List-or-Segment#take-a-random-sample-of-a-list-or-segment1

That's also a good idea, but will taking a random list like that lead to duplicates for another random list later?

Could happen, just use the same segment every time.