In Timeline you have a limit to the number of Technology Units (TU) you have available. Sorcery is considered a "technology" in Timeline, and mages are a TU. Each government has a limit to its number of TUs (excluding peasants) and military units (MUs), depending on its level (era). For example, the average medieval military government has a 40 TU max and an MU max of 15. (The methods of calculation are very simple and are very similar to other d20 methods of character creation.) There is no limit to the number of DUs except for food, which requires land, and in d20 fashion as you increase in size you decrease your AC. The logic behind the TU limit increase is that older civilizations (Rome for example) had a lot of citizens but the vast majority were peasants.
Thus in Timeline the number of mages is limited by a nation's era. They are also limited by the number of spell components a nation has; component availability comes from a nation's trade, colonization or leadership (leadership develops heroes and heroes can do just about anything).
A military unit assigned a mage unit could deal additional fire damage and/or flight. This could effectively introduce low powered bomber planes into a medieval era. Add improved invisibility and you have stealth bombers.
However, you could also have high level clerics in your military units, effectively introducing fusion era (i.e. Star Trek like) medicine into the campaign. Add resurrection, fire resistance, undead armies, summoned monsters and, well, things do tend to look quite different than traditional warfare.
In Timeline you can tweak the level of fantasy by changing the availability of spell components, cleric to population ratio, and high powered denizen frequency for the campaign world you compete in.
http://www.d20timeline.com