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There is a difference between Ashkenazi and Sephardi practice regarding the timing of the bracha (blessing) recited on mikveh immersion.
Sephardi practice for immersion follows the standard pattern of reciting a bracha on a mitzva before performing the mitzva. A Sephardi woman recites the bracha just before she enters the mikveh water, while wearing a robe. She then removes the robe and immerses.
Ashkenazi practice is to immerse once, then recite the bracha (while still standing in the water), and then immerse at least one more time. Delaying the bracha until after a first dip aligns mikveh immersion to become tehorah with immersion for the sake of conversion. Converts cannot recite the bracha over immersion prior to immersing, because they are not Jewish until after they have immersed – and Ashkenazi halachic tradition applies this protocol to mikveh immersion for any purpose. When a woman immersing to become tehorah dips again following the bracha, that parallels the usual order of a bracha preceding mitzva fulfillment.
There are a variety of customs for the total number of immersions: one immersion (Ashkenazi women in a pressing situation may also immerse only once), three times (common Ashkenazi custom), seven times (common Sephardi and Chabad custom), fourteen, and even 49 times – expansions of the typological number seven. We have not heard of a custom to immerse 99 times, but we are not experts in kabbalistic customs, and such a custom may exist. Additional immersions do not affect the timing of reciting the bracha.
Please write back with any further questions.
(For sources, see Yoreh Deah 200: Tur, Beit Yosef, Shulchan Aruch, and Rema.)
This response was updated on 30 May, 2024.