Yes, there are two ways:
2. Random Sample at 20% to 5 existing static lists. This will automate it partially.
Do you know where you create the Random sample?
I know of another company that's been using Marketo for year to do this exact same thing (though their numbers are number higher). And unfortunately there is no automated way to do this.
Here is a related idea you should vote for:
Random sample to a fixed number of records
And here another one - that should be merged with the first:
Cap Affected Number of Leads
@Amy, you can select Random Sample in any flow step condition. E.g. Choice 1: If Random Sample is 20%, send email 1, etc.
Thanks Charlie L - do you still have to create the static lists that are mentioned in the Marketo guide?
Depends on your reporting needs. You don't need it just for sending the emails, but if you want to know exactly who was sent what, you need to either create static lists before the send or you can also create smart lists after the fact.