Echoing what others have said, particularly @loyallyroyal and @Gower.
In an idealized version of your story, you might think the more the better. And then you start writing/coding and then you realize it’s a lot more work than you initially thought. Up to you if you want to pursue this and if you change you mind, you can always go back and edit things.
This is what one published author had to say about his experiences with one of his stories where he had multiple ROs and how it affected his sales. Might offer you some insight into what others’ experiences have been.
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Like I said before, it’s your story. Do what you want. If you want to have a gender locked story that features all homosexual males as RO options, go for it. If you want to have a gender choice story, with all female ROs, go for it.
Honestly, I think you’re focusing on too many of the fine nitty gritty details before you have started writing the scenes since, like Gower said, these projects balloon faster than anyone expected. It’s pretty common to hear about how other authors started planning for a 200 word scene and then the end product somehow become 4 scenes with a word count total of 8,000.
To answer your specific questions, though? Here are my thoughts:
You do whatever you want. It’s your story. If there’s anyone who should be satisfied with it, then it should be you. Write what you want to write and read. There’s an audience for every kind of niche thing out there. While it may not always be as profitable as writing for other audiences, you’ll be satisfied that you wrote for yourself.
As a reader, my only opinion is this. I prefer set genders for characters, regardless if they’re ROs or not. Having to come to a screeching halt in the middle of the story and select the gender of a character is immersion breaking. If you chose to have this as a feature, I’d like to just set the genders for the characters before I even begin reading so I’m not interrupted.
Just like loyallyroyal said, if you have more than 5 ROs (in my case) I honestly just zone out and focus on maybe one or two and ignore everyone else. I like smaller casts because it means that I don’t have to focus on remembering who’s who and don’t ever get whiplash when my MC is suddenly familiar with an NPC/RO that I literally haven’t interacted with at all as the reader.
Smaller casts means that as an author, I can dedicate more effort to each character as well. Not spread myself thin across a much larger cast but not offer as much depth because I’ve gotten burned out by providing only the minimum.