In OVA 4, it is indicated that Seina is married to all eight girls. But then OVA 5 muddies the water.
we have no concrete idea if he marries the girls at the end of the renza ark yet. but yes you are right it sure does.
(Back after a hard drive tragedy)
As far as I can recall from the novels the ones said to be Seina's wives in marriage are Kiriko, Amane, Ryoko B., Neige, and (cabbit by participation) Fuku. Miki, D, Kirche (a.k.a. the rest of the Dual! girls) are cabbits so at most all of them are also cabbits by participation during the first ceremony. Note that this ceremony never really finished as Seina was kidnapped by the Renza Court Ladies (Karen, Suiren, Gyokuren, Hakuren). As I don't have a text file copy of the last GXP Novel volume yet I can only go by the post-Renza-time-skip scene in the GXP anime where it was implicitly shown that apparently Seina married the Renza Court Ladies as well (making 8 wives in the first ceremony + 4 cabbits). This fits in well with the stuff shown on OVA 5.
As for other wives in line for Seina, it was explicitly implied via various characters' dialogues that Seina will be made to marry more and more "volunteers" women because it will allow the many political bodies in the galaxy to monitor his existence well.
The most obvious ones "waiting for the next few rounds of marriage ceremonies" based on their dialogues in the novel are: Tatsuki Ringo, Minaho, Kuramitsu Mikami, Tsukiko Masaki, Kuisu Panta, Kagami-Seto (via Kuisu Panta).
The ones implied who are going to be "made to marry" Seina (a.k.a. being officially "assigned" to deal with duties regarding him) are Minaho's Court Ladies: Mizune Masaki, Otoka Masaki, Fuuka Masaki. And Mikage (who is still a baby in Jurai) is also "reserved" to marry Seina in the future. As of OVA Dai Yon-ki Mikage and co. have stated they aren't allowed to publicly and officially meet Seina yet (although they are already in charge of his finance management in the galaxy) because it is still not publicly known in the Galactic Federation that Yosho and the Masaki Family of Earth are actually alive. They will be given permission to directly interact with Seina after Yosho's survival declaration and the inclusion of the Masaki Family of Earth to the Royal Houses' ranks.
Apart from them, the rest of the ladies implicated to sooner or later also get a piece of Seina are the Renza girls Sojyu, Kaguya, Airen, (Souren??? Daga otoko da.), Jovia, Maya (just wants the DNA), Makihi (Maya's mom so maybe after DNA ONLY as well), Acha (still a kid in the novels but she's a living sacrifice to Seina and JINV so she can't go home to her planet and stays with Seina), and really, just hints and bits and pieces blended in through the novels' dialogue so it's hard to really count the rest. JINV's humanoid terminal and the Philosopher's Stone's humanoid form might also be included there for all I know but that's going to at least be hinted by the last volume still not out. It has been confirmed by Yosho though that Airi does want a piece of Seina which is why she keeps on bothering him, and Yosho has stated that she will probably mellow out if Seina gives her what she wants. He apparently doesn't mind taking care of the resulting offspring back on Earth (that's some complicated husband wife understanding there).
