I think it must be two? clearly if you divide the sphere into stereographic projection from upper and lower part of the sphere i.e. say (1,0,0) and (0,1,0) to make an atlas of sphere.
Why can't you just naively take the same chart and claim it is an atlas of real projective plane?
However, he said you get in a big problem.
But, my lecturer says you can't do that and it must be three charts are needed.
Well, in general why shouldn't one chart cover real projective plane? the point is that opposite points get identified. So you can imagine being an ant and just navigating the space by taking everything you are in z is negative just treating it like z is positive, hence being in the chart of the upper hemisphere?
Is the minimum number of charts a topological property?