I definitely wouldn't advise to go that way. It'll take a tremendous amount of time, increasing exponentially: 1 new round of dialogue per girl = 100 new rounds if you have 100 girls... That'd have been 100 new rounds for 1 girl, or 20 for 5 girls. And considering all the different branches that will likely be available, it'll be such a massive endeavour, it'd be a few years before the first girl is finished. That's why my advice would be to focus on a core group of girls for now, flesh them out, develop them fully, give players a taste of what to expect with new characters.
Another pivotal point to consider is one you brought yourself: modding. The community can (and will) add their own girls, so quantity won't be an issue. But to do so in an enjoyable way for players to read, they need full templates to guide them to take care of the quality part. For instance, how will real-world encounters be managed (if at all)? What about cheating? What about trying to open up a girl to welcoming a new girl in the relationship (e.g. accepting Rebecca back while dating Isabel, then convincing Rebecca to welcome Isabel, and Isabel to accept it)? Can the girls interact with one another? Will there be group chats then? Etc etc... So many unknowns that would require your input to give a solid base. Not to mention if there are interactions between girls, the more girls there are, the more interactions you'll need to code and write.
Lastly, with so many girls to write dialogues for, they won't feel special anymore. They'll all be one among many, and if the player is chatting up dozens of them, it'll break the immersion as they'd all be waiting for a reply while you're busy with one. It'll also burn the player out with so many choices... and burn you out with no end in sight.
Hope you'll at least consider. Good luck nonetheless!