I am not 100% sure I understood what you ask, but if you want to know how many
fullscreen resolutions can a monitor support, try this:
// inside a class:
std::vector<sf::VideoMode> mVideo_Modes
// In a function:
mVideo_Modes = sf::VideoMode::getFullscreenModes();
This will give you the list of all resolutions
and all bits per pixel (8-bit, 16-bit, 32-bit) a monitor can support.
( you might do a bit to kick out all 8-bit and 16-bit resolutions
)
If you have a windowed screen mode, then just make sure that the width and height of the window are not greater than the desktop resolution.