well for starters, the fight value isnt really useful as ur elves are better or just as good as most captains, and if you are fighting someone like a captian, then there is a fair chance that he could kill ur precious guard.... so let ur capt take the likes on them on and have spear and pike support, a sure win against captains (although any evil thing with f6 is really dangerous and should be dealt with care (as in shooting it to pieces and piling on ppl
))
so the reason i would take them is for the pike support, which is really good for elves as it allows you to bring more attacks against your foe (when you get to 3 attacks for a regular soldier you really see that f5 shine) and is sooooo good when used with heros
so after all that the simple answer to your question:
if you are going against an enemy that has only a few big hitters(like goblins), then take only a few (like 2-3), if you are going up against armies with more dangerous stuff (i dont know.... like if they had f5 captains and spiders ect...) then mabye 3-5
so never get too many as they are best used to give your army a bit more flexability
lol, such a long answer so such a simple question