Congratulations! I wouldn't register for clothes at all because people will buy whatever they think is cute when it comes to baby clothes. More thoughts:
Maybe I'm strange, but I find anything you have to yank over baby's head (including onesies) a bit annoying. I prefer side snap shirts to onesies and the one piece pj type outfits (especially zip ups) are the best. They're easy to get on and off, and you don't need to worry about putting baby in new pjs before bed (especially if you just changed baby because she spit up everywhere...).
If you have a happy spitter or a drooly baby, get dozens (not joking, DOZENS) of bibs and change those out often to avoid some outfit changes.
Baby clothes are usually sized Newborn, 0-3 (often just listed as 3) months, 3-6 (often just listed as 6) months, 6-9 (often just listed as 9) months, 9-12 (often just listed as 12) months, and so on.
Don't get too much in newborn size as most babies grow out of those clothes pretty quickly. Also, it's ok for baby to wear something that's a bit baggy anyway. If you cloth diaper, the clothes might be a little tighter (especially in the length) due to the extra bulk of the diaper. You can buy onesie extenders too to help with this.
As far as how many clothes to get, it depends on how often you do laundry. But you probably want to plan on 3, maybe 4 outfits per day just in case (or maybe my happy spitter just manages to make lots of laundry).
And overall I would say just stick with soft, comfy stuff. There's really no reason to put baby in stiff, scratchy clothes or dress her like a mini adult.
Oh, and keep in mind many coats can't be worn in the car seat. So for winter, you will want some light layers or perhaps blankets to wrap baby in.