How many sensors is required to monitor a 60u rack for temperature and humidity?

The number of sensors really depends upon what you are trying to achieve. The simplest measure would be to measure the inflow air temperature at the bottom of the rack and the outflow air temperature at the top of the rack. That would give you a good idea of how the servers are affecting the temperatures all of the way up the rack. At the other extreme are the ASHRAE Thermal Guidelines for Data Processing Environments Second Edition that recomends a very high level of sensor density. The high density approach would be advisable if you wish to identify hotspots within your rack, especially useful after equipment moves which may introduce air flows that cause hot spots.