I’ll do the best I can to answer this question, though I’d stopped following the Superman titles a while back.
There’s Linda, the ‘Earthborn angel’/Matrix gal, who’s hung up the big red ‘S’ permanently, it seems.
There’s Cir-El, who I think died in the end of the Futuresmiths story arc, but I could be wrong.
And there’s Kara Zor-El, the reintroduced Silver-Age Supergirl.
So, one.
However, John Henry Irons’s (Steel’s) niece donned a armored battlesuit and battled alongside Cir-El and a girl called, as far as I can tell, ‘Girl 13’ - a magic user. They were collectively referred to as ‘Supergirls’.
Myself, I’m wondering if Girl 13 is any relation to a similarly-named Phantom Stranger ally, Dr. 13.
As for Power Girl… just don’t even worry about it. Her origin’s been tampered with so many times by other characters in the DC universe and writers for the DC universe, that it’s impossible to say. Hopefully they’ll straighten it all out eventually. But both the idea that she may be Kryptonian and the idea she was of Atlantean origin have been described as false.
In John Byrnes’ Generations books (an Elseworlds series), there were three more Supergirls: Kara Kent, the daughter of Clark Kent and Lois Lane, and twin sisters Lara and Lois Wayne, the original Superman’s great granddaughters.