Most universities I've been to have 9 black/white-boards in a lecture hall. No risk of erasing what was just written because you run out of space. In case you

Most universities I've been to have 9 black/white-boards in a lecture hall. No risk of erasing what was just written because you run out of space. In case you haven't seen this configuration, the boards are mounted on rails that allow them to slide up above each other when you're done with one, with three of these set ups next to each other giving you a 3x3 grid of blackboards. Smaller lecture rooms have a 2x2 grid which lasts quite long also.