Why warships cannot simply add more propeller shafts

Why do ships commonly use two, three or four propeller shafts instead of many more?

Because each shaft requires substantial supporting machinery and hull volume, although podded propulsion may permit different layouts.

The visible shafts are only the final part of a much larger propulsion arrangement. Power generation, gearing and transmission machinery consume far more internal volume than the exposed shaft lines suggest. Adding shafts therefore means finding space for the systems that drive them as well as the shafts themselves. Podded propulsion could change that balance and make six or eight pods a practical standard arrangement.