*Membership spots not really limited!
That could be an answer. How many she had left is hard to say.
I would say she only used a couple. Once that we saw when she was a little kid in New York. Then I'm assuming one we didn't see, because Mels was not old enough to have been alive in 1970. And the one that turned her into Mels. And the River Song one.
So I would guess four were used up.