What altitudes do the salt flats tours visit?

The town of Uyuni and the Salt Flats themselves are around 3700 meters (12139 feet) so 1 and 2 day tours are usually around this altitude only.

For 3 day tours you will visit much higher altitudes with the second nights’ accommodation near the Laguna Colorado area being at around 4300 meters and the Sol de Mañana geothermal field that is visited at around 4900 meters (16076 feet). You may also drive over a pass at  5000 meters (16404 feet), depending on the exact route taken, which will be the highest point.