How to set number of troops in Empire total war

In any Total War game you can't really control the number of troops in a unit (except by getting them killed) - whenever a unit is created it will have always the maximum available number of soldiers for this type. However, the units differ between different countries, which also includes the maximum number of soldiers.

In wiki you can read that:

Every major faction in the game has slightly different line infantry. The British, for example, have some of the finest all-round line infantry in the game, while Russian line infantry have very poor shooting characteristics but have unparalleled charging bonuses. The Ottoman equivalents of line infantry are the Isarelys and the Nizam-I Cedit infantry, while the Maratha Confederacy have Bargir Infantry and Sikh Musketeers.

Austrians have the advantage of having line infantry regiments with 25% more manpower. For example, on ultra settings a full non-Austrian regiment has 160 men; Austrian regiments have 200, instead. On the other hand, Austrian line infantry have generally very poor stats and are the most expensive to field.

Saying that, you can also apparently control the number of soldiers in an unit in the game settings in the graphic options