What does the term "loading" refers to when we talk about Blade loading and Stage loading in Turbomachinery?
The term “blade loading” is basically the same as “pressure ratio”. A blade with a higher blade loading causes the exit pressure from that blade stage to be higher than if the blade loading was lower. Hence, the pressure ratio (exit pressure/inlet pressure) is higher when the blade loading is higher.

The optimum number of blades is a very complex problem. But, in simple terms, too many blades, and the weight is too much, too few blades, and either the exit pressure of the compressor is lower (which makes the engine less fuel efficient), or the pressure ratio across each blade is higher, and the engine is much more likely to suffer compressor stalls. The fuel control may have to allow only a very slow acceleration and deceleration rate, to avoid stalls. It may also stall more easily in crosswind situations. Neither of these are ideal in a take-off situation. You don’t want the engine coughing and losing thrust when you are past the point of aborting the take-off, but aren’t high enough to fly a circuit and land.