The only acidic form of Vit C is Ascorbic acid which in high doses causes stomach upsets and even gripes.
The best and non-acidic form is Ascorbate in the form of Magnesium Ascorbate or Sodium Ascorbate which is non-acidic and does not upset the stomach in high doses.
The bowel tolerance test is the most reliable way of testing if you are receiving enough of the Vitamin: dosage can vary widely, depending on deficiency; anywhere between 1000mg & 20,000mg per day.
Can recommend the most authoritative book on the subject: "Ascorbate: The Science of VitaminC" by Dr Steve Hickey, a member of the board on the Orthomolecular Science team.