[Dimaz]

I think it's one of the reasons they put 'S' in "TBS". I mean, it's one of the strategic choices that largely depends on the game situation and not on some rule like "always use 2 mages for 40 infantry".
So, if you look at it strategically, you should first think about what options in dealing with current opposition you have with current researched spells and with some more advanced ones. If for example you can deal comparable damage to enemy with 10 mages now and with 2 mages after 5 turns of research of those 10 mages, I think it's better to wait for research. However if you will be defeated after 4 turns, this is clearly bad advice. So, as I said, just try to think strategically about your game situation and different options.