[Tuna-Fish]

What do you mean with "as appropriate"?

Imho, with most nations, on turn 10 you should have 9 mages. After that at least one mage/turn until you get another castle, after which it's 2 mages a turn.

Magic and research is the essence of winning anything but the very smallest games, so having as huge mage pool as possible is a rather damned good idea.

So, what mages to build?

I personally first make the ones that can help my early game (on pythium cheapest theurgs with astral magic for BE, etc), then for those with randoms so i can get the useful combinations, then enough national mages so i have the combat mage power i need not soon after(like arch seraphs for caelum, or the cheaper theurg for pythium), and then Last just all sages so I can quickly get the spells I need. Also, 'cos sages cost so little, purchasing them while going for other targets is also a good idea.