It will take quite a bit of research to pin this one down, because searches such as: "incarceration statistics" women "anti drug" (0 hits) and "incarceration statistics" women conspiracy (4 hits) in usa.gov do not seem at all promising. usa.gov includes MANY state government as well as federal government reports, including congressional hearings, corrections department reports, state and federal government studies, etc. The search: "prison statistics" women drugs gets 29 hits, and might possibly lead you at least to agencies or people who are interested in the general subject of women prisoners.
The Department of Justice Bureau of Justice Statistics has many pages of information, but I find that its imprisonment statistics do not break crimes down nearly to the detail required here. At best you may find broad categories such as "Drugs" "Drug Offenses"; maybe "narcotics trafficking" or "drug trafficking".
I think that to find this specific statistic - even if there is a report in the Bureau of Justice Statistics that just happens to mention these yearly figures for women imprisoned for this one offense - it will be best to try first in a commercial database on-site at an academic library, where you can search the full text of journal articles. For example, Academic Search Premier gets 16 articles for the TX (All Text) search: women and incarcerat* and conspiracy and Anti Drug Abuse. Quite possibly some of the cited references in these articles will lead you closer to studies that might have been done.
Even if you get no closer than figures for numbers of women incarcerated each year under, say, Federal drug laws in general; this might lead you to authors working on the problem area of mothers in prison, or doing studies of casual friends who get imprisoned simply because they were caught in the same car with a drug dealer, etc.
The TX (all text) searches, just like Google Books, can be very frustrating, because they will get many articles that mention the words VERY far from each other - in totally unrelated contexts. But they can sometimes help locate quite obscure facts, if you can devise a search that gets only a few dozen promising articles and eliminates hundreds of clearly unrelated articles. I would also certainly try many related words such as: detain*, imprison*, inmates, prisoners, "jail population" etc.
