From Yahoo! support (emphasis mine):
Hello Jeromy,
Thank you for contacting Yahoo Fantasy Sports. I appreciate your
inquiry regarding keepers. I'll be happy to clarify the situation.
There is no ability to specify how many keepers can be kept. It will be up to you as commissioner to ensure the appropriate number of
keepers are kept for each team using the approve keeper players
tool.
When a league declares a keeper declaration deadline, once
that deadline passes the commissioner can use the Confirm Keeper
Players tool to review the keepers submitted by each team, change them
if needed to comply with league rules, and eventually confirm that the
keeper declaration is complete, publishing the results.
Please note: The commissioner is required to complete this process prior to a
scheduled live draft, or the draft will fail to run. If the
commissioner changes any of the team's keepers, this change will be
noted in a transaction to improve league communication and discourage
unethical changes by the commissioner.
Please note: If your Keeper
Declaration Deadline is in the future, or if you have not finalized
your team list, you will not be able to confirm keepers. To approve
keepers:
- Commissioners should log in to your Fantasy league.
- In the "Fantasy" tab drop-down menu, select the league you want to work on.
- Select the Commish Tools tab.
- In the "Keeper Management" section, select Approve Keeper Players.
- Review and edit the keepers selected by each team. To edit a team's keepers, click the Edit Keepers button beneath the team's roster.
- When you are done reviewing and making any changes, select Approve All Keepers and proceed to edit your league's draft. Note: The Confirm
Keeper Players tool is available until approximately one hour prior to
the start of your draft. Additional information on this, and other
Yahoo products, can be found on the Yahoo help page.
If you have any further concerns regarding this issue Jeromy, please
let me know and I will be happy to assist you.