It doesn't look like the information about posts being migrated (PostHistory.PostHistoryType=17) is stored in the data dump. The best data we have is is in the migrations section of the 10k tools

In the last 90 days

    SU        323
    SO         77 
    U&L        44
    DBA        33
    Apple      32
    Webmasters 27
    AskUbuntu  20
    Security   17
    mSF         1
    
    Others     12 to 6 different sites

-- Edit Zypher --   
All time data from a "magical source"

    superuser.com		        	2943
    stackoverflow.com		        2799
    webmasters.stackexchange.com	238
    unix.stackexchange.com		    140
    dba.stackexchange.com		    80
    askubuntu.com			        76
    apple.stackexchange.com		    69
    security.stackexchange.com	    51
    meta.serverfault.com	     	34
    webapps.stackexchange.com	    25