I'm not a fan of exact measurements, and it pains me to disagree even slightly with Chef Tod, but your intuition that rough proportions do count is correct.
Very roughly about 4 cups of mirepoix per 2-1/2 - 3-1/2 gallons volume (10 - 14 cups -- including meat and bones.). For most home cooks using average size vessels and veg, that works out to one large onion, one celery stalk, and one large carrot (not to mention, one chicken!) per average home stock pot.
You may want to alter your mirepoix proportions for any number of reasons. For instance a "blanc," which is white chicken stock, is made with no carrots, in order to keep the stock very lightly colored. On the other hand, roasted chicken stock often has extra carrots to keep the stock looking more chicken than beef.
You may also want to roast your mirepoix on occasion -- for a different, more intense flavor. This is something one (nearly) always does with beef and brown stock.
As long as we're on the subject of chicken stock (don't know why I brought it up), here's an important mirepoix tip: Get the chicken in the pot without any mirepoix or herbs and bring it to the simmer. Skim the scum and discard. Give it another five minutes or so to see if more scum develops. If it does, allow more time for the scum to fully develop, then skim and discard. Now add your mirepoix and herbs. (This technique prevents the vegetables from being contaminated by the scum, and from being removed during the skimming process.).
If, after all the flavor is taken from the chicken, concentrate the stock with the mirepoix in, but the chicken removed. This will result in a much better, rounder flavored stock.
Finally, when you strain the stock, press the vegetables in the sieve to make sure their essence goes with the stock.
Don't let it boil,