25 would be much further than the person I was responding to figured it would go (I.E. "double digits"). I think 25 would probably be possible, but agree that 100 seems pretty unlikely to happen. Definitely seems like a stretch, both for the manga and for the anime.
That said, I'm not clear on why you think that just having more characters voiced is going to enhance the production workload/logistics significantly, irrespective of the number of voice actors. I don't see a reason why that would make their logistics twice or three times as difficult. Or even what the baseline you're comparing it to is.
Actually, I think this series, even with 100 GFs, would still potentially have a smaller cast than a lot of other series do. Admittedly, the ones I'm thinking of are all pretty big names, but still, even twenty five individually voiced characters isn't outside the realm of possibility. Plenty of series churn through large numbers of characters. 100 GFs would just require them to appear more frequently over a longer period of time than any given villain-of-the-week/arc does, true. But the number isn't the main issue, I don't think.
While its true that many shows over their run have hundreds of characters and even potentially many with recurring appearances a majority of them will not show up all in the same episode together all at once and each having their own lines.
There are technically 101 and "Main" characters in 100 girlfriends.
Thankfully the mangaka are smart in not always having all of them constantly interacting with every other all the time, instead saving that for larger plots and arcs, instead focusing on smaller personal one-on-one or small group interactions to particular situations. But large scenes involving all or most all its character happens way more often here than would normally happen even in other long and large character cast shows.
I'm curious to see how they continue to deal with it, and actually hope they do go all-in and adapt it all the way, but won't be surprised of they start cutting corners like doubling, tripling, up in VA roles, or other parts of the production like animation quality after using larger budget on the earlier season to get the fans on board who will be less likely to drop support than if they started more budget conscious for the shows long term future from the get go.Ultimately that depends on fan engagement and monetary support to keep the financial incentive there to keep making it, especially at the same quality as they have so far, which while not top tier was very good and great in some scenes.
Sorry for ranting. I'm not trying to be negative or anything, or put you statement down. Just overthinking and spilling my brain out on the matter with little filter.
TLDR; very ambitious production in the long term, very complicated, many potential problems, likely more than other large cast longer running shows. Worries on maintaining quality throughout and potentially just axing it at one point.
But wanting it to succeed, wanting them to make it succeed, as much as it possibly can because its an ambitious complicated unique property.