For whatever reason sometimes it takes one tablespoonful and sometimes more. I just do one tablespoon and wait 3 hours or so. That is usually enough. If nothing happens, then I do it again. Be sure there is a bathroom nearby.
I usually add baking soda because I do not care for the taste of sour. I also add some type of flavoring to make it easy to take.
If you can get some sodium ascorbate powder it already has the baking soda.
If you do add baking soda to vitamin C, do it slowly and only a little at a time. It will bubble up a lot.