From an answer I wrote at my science website, to a question about why some vaccines need multiple doses to be effective....
After the vaccine has entered the bloodstream and its antigens have caused the body to recognize an invader, the production of antibodies begins. With a vaccine that is weakened form of the virus, these weak buggers can replicate and prompt the body to generate plenty of antibodies. After the action is over, a small number of cells that would recognize the invader remain and circulate throughout the bloodstream; these are known as memory cells.
But some vaccines just have a string of virus protein (like the spike protein of coronavirus) to generate antigens, and so the vaccine itself doesn't continue to replicate in the body, and only a small number of antibodies are created. And very few memory cells are created.
So dosage depends on how robustly the vaccine will cause the production of memory cells. The trick is to have enough antigens in the dosage to generate memory cell production, but not so many as to cause actual illness. Sometimes the best approach is to underdose a couple times, rather than give too much at once. So the end result is you don't get sick, and you have sufficient memory cells to handle the real bug if it shows up.
So, to get back to your question [he was asking how many doses of the Oxford vaccine he would need], I would imagine 2-3 doses is likely. Since the vaccine in question uses the weakened form of a common cold virus as a transporter of a piece of code of CV spike protein, it will replicate somewhat, but apparently not enough to proliferate in the bloodstream and prompt your system to build a full immunity in one go.