In theory it's a nice idea, but it means less time to flesh out all character story arcs, so i'd vote 4 as maximum...
Four doesn't give equal options to everybody, though.
They've already said some of the LIs are exclusively lesbian/gay (let's say, one man and one woman). If there's only four characters in total, then the other two have to be bisexual options.
That leaves only one straight option per gender, which is completely, absolutely never going to happen. They'd never give male Inquisitors two men to romance and only one women, nor female Inquisitors two women to romance and only one man.
I don't really like jumping to assumptions, but six LIs (two same-sex, two heterosexual and two bisexual, one gender of each) really is the minimum number that guarantees two things:
1. More than one option for everybody. Both straight and gay Inquisitors get two options, in total each gender of Inquisitor can choose from four LIs (everybody except the gay LI of the opposite sex and the straight LI of the same gender).
2. Equal options for both same-sex and opposite-sex romances. They both get two characters. I suppose bisexual Inquisitors can in theory choose from all four available LIs (of the six), but surely not at the same time.