Ok this is how it goes. Lets say in some Russian town is Russian first infantry division. The size of the 1 division is 10000 men. And now there are 4000 men in the barracks of the 1 division’s the 6000 if waiting in reserve. Witch means that the surtan % of reserve troops will be deployed in the units and the rest in T.D.(territorial defense).The size of Russian army is 4 million troops(active +reserve).The T.D is also reserve but there are no units to deployed them in.

In Serbia there are 8 million people. From witch 5 million are over 18.Les then half are men. So 2 million-people over 60=1,25 million men over 18-55.They all served in army.
All males are obliged to serve the army by the law.