Once all library technologies have been researched, Scholars- and Knowledge- are essentially useless, doing nothing more than taking up valuable population room, unless you want to create Nuclear missiles and construct the Super Collider and the Space Programwonders. When more room is required to raise an army, your scholars should be the first to go.
The number of Scholars within a University is shown in the building graphics of the University. Each University can hold up to 7 Scholars.
Like Citizens, Scholars can be converted into Militia once it is researched at a Tower. This is unlikely to be used for defensive purposes because Scholars automatically return to their University when they are ejected, and thus a bit difficult to convert. However, if the city is taken over, turning the Scholars into Militia may buy them extra time to escape, since later-age Militias have more hit points.