Use the Send Email condition in your flow step. Then click Add Choice. Now with the choices, you can get as granular as you want. My 2 favorites are alphabetical and random sample.
Example conditions for flow:
If email address starts with a; b; c send email. Default Choice: do nothing
wait one hour
If email address starts with d; e; f send email. Default Choice: do nothing
wait one hour. yada yada yada
or
If random sample is 25 (representing 25% of the whole list) send email. Default Choice: do nothing
wait one hour
If random sample is 25 send email. Default Choice: do nothing
wait one hour
If random sample is 25 send email. Default Choice: do nothing
wait one hour
If random sample is 25 send email. Default Choice: do nothing