Difference between "magnitude" and "multitude"
Solution 1:
Magnitude
Definition: Extent of dimensions; size; applied to things that have length, breath, and thickness.
Multitude
Definition : The state of being many; numerousness.
For more details you can follow these links -
Link 1 : Magnitude
Link 2 : Multitude
Hope it'll help :-)
Solution 2:
In the example sentence, magnitude is used to refer to size (how big) while multitude is used to refer to numbers (how many).