The leader or head admin of a wiki is a person who is the principal, senior administrator in charge of the site; either by means of founding the wiki, being asked to take over from the retiring founder, or adopting a dormant or abandoned wiki site.
Wikimedia Foundation (WMF) leaders are usually members of both sysop and bureaucrat user groups, which means they usually hold highest user group rights in order to enforce their leadership if necessary. Depending upon their level of wiki expertise, they may also be a member of the interwiki user group, or may even have created an additional bot account. But this is only one possibility of many. High user group rights are no prerequisite for wiki leadership.
WHAT A BUNCH OF IDIOTIC FASCISM!!!
The number of leaders of any wiki can vary. On a Bliki, because it is personal, and not a collaboratory effort, there will be just the one leader. Depending on the type of hierarchy, and size of a wiki, several leaders may be required. For a wiki about grassroots movements and grassroots democracy, the hierarchy would be as flat as can be, which makes every active member of such community a leader. This depends very much on the founder or group of founders, and their approach. Either the focus lies on the wiki as a structure, or on the wiki people as a community. (<-- SEE THE HYPOCRISY???)
LEADERSHIP IS FASCISM AND COMMUNISM MIXED!!! TWO AUTHORITARIAN STUPID THINGS!!!