A female imam is a woman who leads others in prayer. Her question is akin to asking how many women drink coffee. Her arrogant ignorance on the nature of prayer leadership--it is both practical and institutional--is sadly typical. A thread.
Practically, when a woman leads anyone in prayer, she is an imam. Women lead their children, they lead family members and friends (including men), they lead other women in and outside the home. Far less often, women lead in institutional contexts (women-only or mixed-gender).
Institutionally, it is now widely deemed legal for those communities who want a female imam in their mosque. There's been a lot of legal analysis around this (some of which I took part in with @egypsci). I was an institutional imam for years. papers.ssrn.com/sol3/papers.cf…