To be precise, the flyback transformer produces a sawtooth waveform, which generates the raster scan (lines are actually a bit slanted, dropping from left to right). While there's a preset frequency, a TV set will synchronize with the input signal – and there's some robustness in this. E.g., most 50 Hz CRT TVs will happily display a 60 Hz signal (as may be seen in the PAL/60 pseudo-standard). Horizontally, screen activation is directly bound to signal intensity, hence "analog TV", therefore only limited by the signal response of the phosphor used and by the amplification circuitry (for a character display, you'd want your signal ramps not to become too shallow). Also, there's some flexibility in the overall number of lines displayed. (E.g., while there are nominally 262 scan lines in a NTSC field, Atari VCS games historically varied between 238 and 290.) There are no hard specs for a monochrome tube, especially, when driving it directly, w/o going through modulation/demodulation stages. However, there will be a visible vertical separation, if you're just using odd fields as in non-interlaced video (due to the sawtooth waveform generating the raster scan), resulting in visible scan lines. Notably, most of the constraints resulting from this can be addressed by the type design of the display characters.
