Q: "The number of field lines determine the magnitude of strength of electric fields, why is this? "
A: It is by definition that the more field lines you have, the stronger the electric fields. It is just a way for us to show to the marker that there is a stronger electric field.
Q:"Why is the positive plate the high potential plate?"
A: Once again this is by definition. It just happens to be that the plate with the greatest potential is positive. We could say the negative plate will have the highest potential but the physicists worldwide have defined the positive plate with the greatest potential so we should stick to their model, otherwise we'll get into trouble with them