On Sleeper, there are two kinds of drafts:
Startup draft:
Rounds based on your league's roster size and taxi squad settings
Supplemental draft:
Customizable range of 1-10 rounds
Leagues in their first season will have a startup draft, as will future seasons for redraft and keeper leagues.
Dynasty leagues will only have the option to run supplemental drafts after completing their initial startup draft.
If you need to adjust the number of rounds in your startup draft, you'll need to change the size of your rosters.
If you need more than 10 rounds for your supplemental draft, you can set up back-to-back drafts. Only one draft can be created at a time, but there is no limit to the number of supplemental drafts you can run.