In Sleeper, there are two kinds of drafts. A typical 'startup' draft, that has rounds based on your league's roster spots and taxi squad, and a 'Supplemental' draft, that has a customizable range of 1-10 rounds.
Leagues in their 1st season will have a startup draft, as well as any future seasons for redraft or keeper leagues.
Dynasty leagues will only have the option to run supplemental drafts once they use the initial startup draft.
If you need to resize the number of rounds for your startup draft, adjust the size of your rosters.
If you need more than 10 Rounds for your Supplemental Draft, you can set up back-to-back drafts one after another. Only one draft can be created at a time, but there isn't a limit to how many supplemental drafts you can do.