AdminCP -> Options -> User Options -> Members to List Per-Page.
AdminCP -> OIptions -> User Options ->Maximum Usernames in Members Online Now Box
As per their descriptions:
Limit the number of members to show on each page of the registered member list, and online members list.
In order to prevent the 'Members Online Now' becoming enormous on a very busy board, you may limit the number of names before a '... and 12 more' link is added to terminate the list. A value of 0 disables the limit.
Keep in mind that increasing this might impact performance.