When to buy/how to store/ how much--sparkling grape juice?

How much you need really depends on your guest list, but I would honestly not buy more than 4 or 5 bottles. I've never seen sparkling grape juice refrigerated at the store, so I would assume that it's shelf stable until opened.

View Quoted Comment That makes sense. I don't want 20 bottle for sure, but I'm just trying to figure out how else to make sure each table has some. So I guess I could look into decanters? That's why I was thinking maybe smaller bottles would be better. We invited about 173, Rsvp's aren't due until next month so the final number is a bit in the air at the moment

Yeah, sparkling grape juice has a pretty stable shelf-life, so you can buy it whenever you want and it shouldn't change. I'm not sure what you mean by decanters, but as a carbonated beverage, as soon as it's opened it will start to go flat, so it shouldn't be opened until right before serving. You can buy the bottles that are about the size of champagne bottles. Each should hold about 4+ servings. I'm not personally a fan of sparkling cider/grape juice, so I'm not sure how much your guests will drink. If you think they'll drink it as a beverage (not just a quick sip for a toast), I'd probably figure about a half bottle per person. For toasts, I'd guess one bottle for every four people will be more than enough. I would assume it will definitely taste better chilled than room temperature, so I'd plan to refrigerate the bottles a few hours before they are served and/or store them on ice.