If someone was not aware of the ruling on spitting out food that was stuck between the teeth and would swallow the food on occasion, but now they are aware of the ruling, do they have to keep qadha fast of all years as a precaution as they do not know when they would have swallowed something or not?

If he is certain he did it, but does not know how many times, and it is between two numbers, he can build on the lower number.