Right now I see 5 automatic preliminary entries for Hanazawa (Kanade, Ruri, Charlotte, Nadeko, Kuroi Mato) and 2 for Kugimiya (Shana, Taiga) (Nagi and Louise still have to be renominated), but there is the potential for each side to have more entries.
Unrelated but interesting coincidence:
Railgun's ISML postseason opponents:
2010 Nadeko (Kana) < Taiga (Rie) < Shana <2x> (Rie) < Mio <2x> (Neither one)
2011 Charlotte (Kana) < Taiga (Rie) < Ruri (Kana) < Shana (Rie)
I doubt it will happen again this year, but I doubt both sides would want to challenge Railgun.