Ascorbic acid is the straight up form of vitamin C that appears naturally in foods. This is not necessarily the best form to take as a supplement as some people can find it irritating to their gut. Sodium ascorbate is gentler and then calcium ascorbate is even less irritating.
There are many theories on high dosing vitamin C to “bowel tolerance” whereby you consume high amounts until you get diarrhoea. This can sometimes be as high as 10g in a day. This level of high dosing is not all that necessary and can just irritate your gut more than anything. Higher dosing of vitamin c is very useful if you have any tissue injury or surgery as this can help in the healing process as vitamin C is great for tissue healing e.g. around 3g-6g/daily.
Making sure your vitamin C also comes with bioflavonoids is key. They are always together in food like oranges, so it makes sense to supplement with both as well.
Green Nutritionals Pure Plant Source Vitamin C powder which uses whole food powders that are naturally rich in vitamin C and bioflavonoids is a great choice. If you're not into powders, Herbs of Gold Vitamin C 1000 Plus is my pick.