While I would suspect multiple characters will always be more popular as often people will always want to see more weight gain content and more characters if you give them the chance. Some things to consider, if this is your first game it might be better to keep the scope smaller as @Lightning has also suggested. More characters will always lead to a larger scope and increased development time. Most commonly people “bite off more than they can chew” when jumping into an early project and there are always going to be mistakes and things that could be done better as you learn and get more advanced in your craft. A smaller scope project that is more manageable might be for the best in my opinion.
Depending on what type of VN or style of game you want to make you can always have a single completed route for a character and go back and add more storylines or solo routes depending on how you plan to structure your game. Even decisions about linear gameplay, branching choices, or multiple routes will affect your scope size and development time. Most devs will tell you to start small as you’re starting out because your scope and time to complete will never be what you plan/imagine it will be when you’re new to it. These would also be useful for a voter to understand to give you better feedback and polling.
Polls can be great to help give you direction or make some decisions you are unsure of but they are probably more helpful when the voter understands some basics about what your game will be. As stated above a single character or multiple could greatly influence your project scope when combined with other factors. For instance Love is the way to my heart had a great concept but it’s scope was way to large and trying to accomplish too much, in my opinion. With 14 love interests at the same time (there were many factors that led to its cancellation of course) but I believe the game might have been more manageable if the dev had focused on one girl and completed her fully with their planned style of development. Since it seems like you would be locked into one girl’s route once you choose her in the beginning.
Just my opinion and some thoughts on the matter, good luck with development on your game, the first step is always the hardest. Great to see you trying your hand and making something.