Monitor frequencies in kHz are horizontal frequencies and only tell you how many lines are being drawn per second. Given 288 lines of resolution (call it 300 to factor in front porch, back porch, and sync) and you get 50Hz refresh rate (or vertical frequency). Most of the time, the number of pixels per line isn't really limited by the monitor (well, at least not electronically, only by the size of the phosphors) but most people like to use square(ish) pixels, so, assuming a 4:3 aspect ratio, that's 384 pixels.