How much can anorexia decrease a woman's fertility after she's recovered?

It may or may not: Anorexia often stops a women's periods. When you do not have your period, then you are not ovulating. Some women w/ anorexia will start having their periods again once their weight is normal, some will not. All women with a previous history should have a thorough evaluation by a fertility specialist to assess their ovarian function and their thyroid function.

Depends: It depends on the severity. Some women recover from the anorexia/eating disorder and gain weight, but may never have periods again. Thus they would be "normal" from the anorexia point of view, but their fertility may remain subnormal and they may need assistance to conceive.