i think my answer to the first question really depends on just how much you're trying to replicate DnD tonally,
Honestly, I don't know. We'll see how it comes out in the end, but so far the votes are suggesting at least a somewhat different direction.
and on the features mentioned in the following questions, i think if you're providing a choosable list of traits then that's serving the purpose subraces were intended to serve for customisation and you'd only need one basic member of each species.
also, on Q3 is that asking the number of traits any given member of a species should have or the number available to pick from in total?
The former.
Currently (and not including things like size, speed, creature type, and languages), dragonborn have two traits (breath weapon and damage resistance), mountain dwarfs have five (darkvision, resilience, weapon training, tool proficiency, and stonecunning), and high elves have six (darkvision, keen senses, fey ancestry, trance, elf weapon training, and cantrip). So as you can see, there's a big disparity in how many traits any one race has. Even if you take out the purely cultural bits (proficiencies, and potentially stonecunning and cantrips), there's still a gap--and dragonborn have
no cultural traits except for language.
I had thought about doing a highly streamlined/"OSR-ified" version of Level Up, in which each heritage would have two traits plus choice of a gift (each of which had one trait in them). It really made me think about what each heritage actually
needs to be iconically that heritage. Which for elves, was going to be keen vision and a cantrip. One of the gift options was fey ancestry; I don't think I found a really good second option).
I didn't really think much else was really needed to make them recognizably elves. (As it turns out, I think Level Up is a bit too complicated to be simplified enough for my goal.)
But anyway, that's what I mean: I think most people either want the races to have a bunch of special abilities, or to be very simple. D&D, currently, leans heavily towards having a bunch of special abilities.