How many glasses of o.j. would you need to drink to obtain 500mg of vit C?

Totally depends on what sort of orange juice. Most orange juice from cartons has been concentrated, and then rediluted to normal volume, and had vitamin C added to it. The amount should be listed on the carton.

If you are squeezing fresh oranges, then an average orange has about 60mg of Vitamin C, so you'd need about 8 and a half oranges for 500mg vit C.