How much can a cup size be reduced to during a breast reduction (J to A/B)?

There's no simple answer to this question because cup size is a very inconsistent way to measure breast size. Breast reductions are measured by the weight of tissue removed, but cup size relates to chest circumference, breast width, and volume. Going to an A cup from a very large cup size would leave a very unattractive breast in cases where the chest is wide.

Thanks for your question. It is difficult to predict cup sizes post-operatively as it is dependent on a number of factors and not just volume (chest circumference and projection).
The limiting factors in terms of how small we are able to make your breasts following a breast reduction are:- the amount of tissue that needs to be left behind in order to ensure that the nipple has an adequate blood supply - the amount of tissue required to make an aesthetically pleasing breast shape.
In some cases where very significant size changes are desired, such as the one you are suggesting, then it may be possible to use a technique such as nipple grafting to achieve the desired outcome, but this may also be at the expense of a natural appearance to the breast.

It is not true that a breast can only be reduced by 1 to 2 cup sizes. Going down from a J cup to an A cup is not realistic since this amounts to such a huge amount of tissue to be removed that an aesthetically pleasing breast shape may not be achieved since it almost amounts to a mastectomy.
When a breast is reduced the nipple needs to remain attached to a pedicle in order to keep the nipple alive. This pedicle will constitute part of the volume of the breast which may be significant.
In general cup size cannot be guaranteed but a significant reduction can be achieved. Usually somewhere in the region of a C cup is realistic since it allows some tissue which can be reshaped to provide a nice shape.