Redundant tag; most posts on Danbooru are focused on female characters. Try -1boy -Multiple Boys or some variant thereof instead:
- 1girl (one female, with or without males)
- 1girl Solo (one female, without males)
- 1girl Solo Focus (one female, with offscreen males)
- Multiple Girls (multiple females, with or without males)
- ~1girl ~Multiple Girls (one or more females, with or without males)
- ~1girl ~Multiple Girls -1boy -Multiple Boys (one or more females, without males)
If you are looking for the Females Only tag as it is used on E-Hentai and other sites, the Danbooru equivalent is simply Yuri.